This provides a minor speedup by avoiding an extra syscall

	require 'socket'
	require 'benchmark'
	nr = 100000
	msg = 'hello world'
	buf = ''
	size = msg.bytesize
	puts(Benchmark.measure do
	  UNIXSocket.pair(:SEQPACKET) do |a, b|
	    nr.times do
	      a.sendmsg_nonblock(msg, 0, exception: false)
	      b.recv(size, 0, buf)
	    end
	  end
	end)

             user     system      total        real
before:  0.330000   0.340000   0.670000 (  0.678235)
 after:  0.290000   0.240000   0.530000 (  0.534527)

* ext/socket/rubysocket.h: flags for common socket families
  (rsock_getfamily): update signature
* include/ruby/io.h: comment socket FMODE flags
* ext/socket/init.c (rsock_getfamily): memoize family
* ext/socket/basicsocket.c: adjust rsock_getfamily calls
* ext/socket/ancdata.c: ditto
  [ruby-core:69713] [Feature #11298]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51097 b2dd03c8-39d4-4d8f-98ff-823fe69b080e