dhcpv6relay: fix build with gcc 9.x

[bkuhls]

Fixes various build errors seen with gcc 9.x:

Making all in dhcpv6relay
  CC       dhcpv6relay_la-dhcpv6relay.lo
dhcpv6relay.c: In function 'dhcpv6relay_down':
dhcpv6relay.c:193:1: error: parameter name omitted
 void dhcpv6relay_down(void*, int)
 ^~~~
dhcpv6relay.c:193:1: error: parameter name omitted
dhcpv6relay.c: In function 'dhcpv6relay_release_route':
dhcpv6relay.c:265:45: error: parameter name omitted
 void dhcpv6relay_release_route(const struct in6_addr* addr, uint8_t prefixlen, uint32_t /* lifetime */)
                                             ^~~~~~~~
dhcpv6relay.c: In function 'dhcpv6relay_process_ia_pd':
dhcpv6relay.c:343:2: error: label at end of compound statement
  default:
  ^~~~~~~
dhcpv6relay.c: In function 'dhcpv6relay_process_ia_na':
dhcpv6relay.c:380:2: error: label at end of compound statement
  default:
  ^~~~~~~
dhcpv6relay.c: In function 'dhcpv6relay_process_packet_for_routes':
dhcpv6relay.c:422:2: error: a label can only be part of a statement and a declaration is not a statement
  dhcpv6relay_route_func func;
  ^~~~~~~~~~~~~~~~~~~~~~
dhcpv6relay.c:449:6: error: label at end of compound statement
      default:
      ^~~~~~~
dhcpv6relay.c: In function 'dhcpv6relay_server_event':
dhcpv6relay.c:463:1: error: parameter name omitted
 void dhcpv6relay_server_event(int fd, void*)
 ^~~~
dhcpv6relay.c:558:2: error: label at end of compound statement
  default:
  ^~~~~~~
dhcpv6relay.c: In function 'dhcpv6relay_client_event':
dhcpv6relay.c:619:1: error: parameter name omitted
 void dhcpv6relay_client_event(int fd, void*)
 ^~~~
dhcpv6relay.c: In function 'dhcpv6relay_send_router_advertisement_timed':
dhcpv6relay.c:795:1: error: parameter name omitted
 void dhcpv6relay_send_router_advertisement_timed(void*)
 ^~~~
dhcpv6relay.c: In function 'dhcpv6relay_router_solicitation':
dhcpv6relay.c:802:1: error: parameter name omitted
 void dhcpv6relay_router_solicitation(int fd, void*)
 ^~~~
dhcpv6relay.c: In function 'dhcpv6relay_up':
dhcpv6relay.c:855:1: error: parameter name omitted
 void dhcpv6relay_up(void*, int)
 ^~~~
dhcpv6relay.c:855:1: error: parameter name omitted

[Neustradamus]

@paulusmack, @jkroonza: What do you think about this PR?

[jkroonza]

Mostly the same old "mark as attribute((unused))" everywhere - didn't mark all of them.

Where there's actual names to be had, use them rather than a generic unused.

[bkuhls]

updated the patch, it was build-tested by the buildroot project toolchains which include an old gcc 9 one (bootlin-aarch64-glibc-old): https://gitlab.com/bkuhls/buildroot/-/pipelines/2615333725

Personally I have no interest in using such old gcc versions, I just try to fix build errors found by the buildroot set of toolchains. If you declare that you do not support such old toolchains I will happily restrict the ppp package to newer gcc versions and this patch can be closed.

[jkroonza]

@paulusmack I'm happy enough, please merge.

I do think at some point we should probably have the leading whitespace conversation ... the shiftwidth=4 + tabstop=8 doesn't seem to display so well in github ...