socket: avoid redundant fcntl on Linux

* ext/socket/ancdata.c (bsock_sendmsg_internal,
  bsock_recvmsg_internal):
  avoid redundant fcntl on Linux
  [ruby-core:69154] [Feature #11145]
* ext/socket/init.c (rsock_s_recvfrom_nonblock): ditto
* ext/socket/rubysocket.h (MSG_DONTWAIT_RELIABLE): new macro

MSG_DONTWAIT is enough to force non-blocking I/O under Linux,
so avoid changing the state of a socket.  This will allow certain
threads to do a non-destructive non-blocking "peek" while others
block (without relying on an extra ppoll syscall).

We shall be conservative about enabling this feature since some
OSes may have incomplete support for MSG_DONTWAIT.  I shall
defer to a FreeBSD expert to enable that for FreeBSD.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50666 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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