Option to handle DNS requesets by lwIP

[skuenzer]

This PR introduces an option to use lwIP's DNS resolution for getaddrinfo(), getnameinfo(), gethostbyname(), and family. It can be enabled if lwIP is available. It will enable lwIP's DNS and socket interfaces.
The advantages of this implementation is that

1. there needs to place an /etc/resolv.conf in the VFS,
2. Works with DNS servers that lwIP got assigned with a DHCP reply.

This PR depends on PR #48 for lib/lwip.

[StefanJum]

@skuenzer I get some redefinition errors when building, am I missing something in the config step?

In file included from /media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:9:
/media/stefan/projects/unikraft/maintainer-tools/workdir/apps/nginx/workdir/build/liblwip/origin/fork-lwip-UNIKRAFT-2_1_x/src/include/lwip/netdb.h:92:8: error: redefinition of ‘struct hostent’
   92 | struct hostent {
      |        ^~~~~~~
In file included from /media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:8:
/media/stefan/projects/unikraft/maintainer-tools/workdir/apps/nginx/workdir/build/libmusl/include/network/include/netdb.h:69:8: note: originally defined here
   69 | struct hostent {
      |        ^~~~~~~
In file included from /media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:9:
/media/stefan/projects/unikraft/maintainer-tools/workdir/apps/nginx/workdir/build/liblwip/origin/fork-lwip-UNIKRAFT-2_1_x/src/include/lwip/netdb.h:103:8: error: redefinition of ‘struct addrinfo’
  103 | struct addrinfo {
      |        ^~~~~~~~
In file included from /media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:8:
/media/stefan/projects/unikraft/maintainer-tools/workdir/apps/nginx/workdir/build/libmusl/include/network/include/netdb.h:16:8: note: originally defined here
   16 | struct addrinfo {
      |        ^~~~~~~~
/media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:13:17: error: conflicting types for ‘gethostbyname’; have ‘struct hostent *(const char *)’
   13 | struct hostent *gethostbyname(const char *name) {
      |                 ^~~~~~~~~~~~~
In file included from /media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:8:
/media/stefan/projects/unikraft/maintainer-tools/workdir/apps/nginx/workdir/build/libmusl/include/network/include/netdb.h:116:17: note: previous declaration of ‘gethostbyname’ with type ‘struct hostent *(const ’
  116 | struct hostent *gethostbyname (const char *);
      |                 ^~~~~~~~~~~~~
/media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:17:17: error: conflicting types for ‘gethostbyname2’; have ‘struct hostent *(const char *, int)’
   17 | struct hostent *gethostbyname2(const char *name __unused, int af __unused) {
      |                 ^~~~~~~~~~~~~~
In file included from /media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:8:
/media/stefan/projects/unikraft/maintainer-tools/workdir/apps/nginx/workdir/build/libmusl/include/network/include/netdb.h:135:17: note: previous declaration of ‘gethostbyname2’ with type ‘struct hostent *(const’
  135 | struct hostent *gethostbyname2(const char *, int);
      |                 ^~~~~~~~~~~~~~
/media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:27:5: error: conflicting types for ‘gethostbyname_r’; have ‘int(const char *, struct hostent *, char *, size_t,  struct ho}
   27 | int gethostbyname_r(const char *name, struct hostent *ret, char *buf,
      |     ^~~~~~~~~~~~~~~
In file included from /media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:8:
/media/stefan/projects/unikraft/maintainer-tools/workdir/apps/nginx/workdir/build/libmusl/include/network/include/netdb.h:133:5: note: previous declaration of ‘gethostbyname_r’ with type ‘int(const char *, stru}
  133 | int gethostbyname_r(const char *, struct hostent *, char *, size_t, struct hostent **, int *);
      |     ^~~~~~~~~~~~~~~
/media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:33:5: error: conflicting types for ‘gethostbyname2_r’; have ‘int(const char *, int,  struct hostent *, char *, size_t,  st}
   33 | int gethostbyname2_r(const char *name __unused, int af __unused,
      |     ^~~~~~~~~~~~~~~~
In file included from /media/stefan/projects/unikraft/maintainer-tools/workdir/libs/musl/lwip/gethostbyname.c:8:
/media/stefan/projects/unikraft/maintainer-tools/workdir/apps/nginx/workdir/build/libmusl/include/network/include/netdb.h:134:5: note: previous declaration of ‘gethostbyname2_r’ with type ‘int(const char *, int}
  134 | int gethostbyname2_r(const char *, int, struct hostent *, char *, size_t, struct hostent **, int *);

[skuenzer]

Ouh sorry, you need to have the following lwip PR also applied. I forgot to mention that dependency in the description: unikraft/lib-lwip#48

[StefanJum]

All good now, thanks.
Reviewed-by: Stefan Jumarea stefanjumarea02@gmail.com

[Mihnea0Firoiu]

All good.
Reviewed-by: Mihnea Firoiu mihneafiroiu0@gmail.com

[razvand]

Reviewed-by: Razvan Deaconescu razvand@unikraft.io
Approved-by: Razvan Deaconescu razvand@unikraft.io