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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