ruby/io.h: get rid of conflict on AIX

* include/ruby/io.h: rename SVR3,4 member names as POSIX compliants,
  to get rid of conflict on AIX.  [ruby-core:53765] [Bug #8174]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39987 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* lib/resolv.rb: Test Windows platform by detecting LoadError when
  require 'win32/resolv' suggested by Nobuyoshi Nakada [ruby-core:53389].
  [ruby-core:53388] [Feature #8090] Reported by Charles Nutter.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39988 b2dd03c8-39d4-4d8f-98ff-823fe69b080e