* dir.c (dir_initialize): use rb_gc_for_fd for ENOMEM
* ext/socket/init.c (rsock_socket): ditto
* ext/socket/socket.c (rsock_socketpair): ditto
* internal.h (rb_gc_for_fd): prototype
* io.c (rb_gc_for_fd): remove static
  [ruby-core:71623] [Feature #11727]

Manpages for opendir(2), socket(2), and socketpair(3posix)
describe ENOMEM as a possible error for each of these;
handle it consistently with our existing wrappers for
open(2)/pipe(2) etc...

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52726 b2dd03c8-39d4-4d8f-98ff-823fe69b080e