Fix emacsclient hang when backgrounded

Fix emacsclient hang when backgrounded

Problem reported by Kaushal Modi in:
https://lists.gnu.org/r/emacs-devel/2018-12/msg00083.html
The tcdrain call replaced an fdatasync call which had no
effect on the tty, so removing it entirely shouldn’t cause
problems.  The fdatasync call replaced an fsync call which
also had no effect on the tty, and the fsync call seems to be
badly-merged revenant of emacsclient’s old (circa 2004) way of
communicating to and from Emacs via FILE * streams, where
fsync was apparently needed when talking to sockets.
* lib-src/emacsclient.c [!DOS_NT]: Don’t include termios.h.
(flush_stdout): Remove.  All callers removed.
(main): Do not drain the tty after "Waiting for Emacs..."
message.  There should be no need to drain, and draining it
might send us a SIGTTOU.  Do not fflush stdout just before
exiting, as exiting does that for us.