Deprecation document for gethostbyname,gethostbyaddr.

[Feature #13097]

I confirmed current ruby (Ruby 2.4 and trunk) uses
gethostbyname() and gethostbyaddr().

Socket.gethostbyname uses getaddrinfo() and gethostbyname().
Socket.gethostbyaddr uses gethostbyaddr().

Socket.gethostbyname uses gethostbyname() to obtain alias hostnames.

RFC 3493 defines getaddrinfo()/getnameinfo() and
describes the problems of gethostbyname()/gethostbyaddr().
The problems are difficult protocol handling and thread-unsafety.

Since Ruby has GVL, the thread-unsafety doesn't cause wrong result.
But it may block other threads until finishing DNS query.

Socket.gethostbyname has the protocol handling problem.
It returns only one address family:

```
% ruby -rpp -rsocket -e 'pp Socket.gethostbyname("www.wide.ad.jp")'
["www.wide.ad.jp",
 [],
 10,
 " \x01\x02\x00\r\xFF\xFF\xF1\x02\x16>\xFF\xFEKe\x1C",
 "\xCB\xB2\x89:"]
```

www.wide.ad.jp has one IPv6 address and one IPv4 address.
But Socket.gethostbyname returns only one address family, 10 (AF_INET6),
which is the address family of the first address.

Also, Socket.gethostbyname and Socket.gethostbyaddr uses
4-bytes binary IPv4 address and 16-bytes binary IPv6 address.
This is not usual in other socket API in Ruby.
(Most socket API uses binary sockaddr string or Addrinfo object)

I think Socket.gethostbyname and Socket.gethostbyaddr are too far
from recommendable API.

So, I added deprecation description for documents for them.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60266 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/socket/socket.c

@@ -983,7 +983,17 @@ sock_sockaddr(struct sockaddr *addr, socklen_t len)
  * call-seq:
  *   Socket.gethostbyname(hostname) => [official_hostname, alias_hostnames, address_family, *address_list]
  *
- * Obtains the host information for _hostname_.
+ * This method is deprecated since following reasons:
+ *
+ * - The 3rd element of result is the address family of the first address.
+ *   The address families of rest addresses are not returned.
+ * - Uncommon address representation:
+ *   4/16-bytes binary string to represent IPv4/IPv6 address.
+ * - gethostbyname() is may take long time and it may block other threads.
+ *   (GVL cannot be released since gethostbyname() is not thread safe.)
+ * - This method uses gethostbyname() function already removed from POSIX.
+ *
+ * This method obtains the host information for _hostname_.
  *
  *   p Socket.gethostbyname("hal") #=> ["localhost", ["hal"], 2, "\x7F\x00\x00\x01"]
  *
@@ -1000,7 +1010,15 @@ sock_s_gethostbyname(VALUE obj, VALUE host)
  * call-seq:
  *   Socket.gethostbyaddr(address_string [, address_family]) => hostent
  *
- * Obtains the host information for _address_.
+ * This method is deprecated since following reasons:
+ *
+ * - Uncommon address representation:
+ *   4/16-bytes binary string to represent IPv4/IPv6 address.
+ * - gethostbyaddr() is may take long time and it may block other threads.
+ *   (GVL cannot be released since gethostbyname() is not thread safe.)
+ * - This method uses gethostbyname() function already removed from POSIX.
+ *
+ * This method obtains the host information for _address_.
  *
  *   p Socket.gethostbyaddr([221,186,184,68].pack("CCCC"))
  *   #=> ["carbon.ruby-lang.org", [], 2, "\xDD\xBA\xB8D"]