buffer fullness is more robust to detect the message is too big for
  the buffer.
  AIX 7.1 recvmsg doesn't set MSG_TRUNC for rflags when MSG_PEEK is
  given.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52673 b2dd03c8-39d4-4d8f-98ff-823fe69b080e