Create gdb_termios.h (and cleanup gdb/{,gdbserver/}terminal.h)

As requested, I'm sending this as a separate patch because it is ready
to be included as-is.

The idea here is that both gdb/terminal.h and gdb/gdbserver/terminal.h
share the same code, which is responsible for setting a bunch of
defines on based on the presence of termios.h and a few other headers.
This simple patch just moves this common code to common/gdb_termios.h
and makes the necessary adjustments on both GDB and gdbserver so that
they can use this new header.  It also implements the some header
checks on common/common.m4.

As a bonus, gdb/gdbserver/terminal.h can be removed because it's now
empty.

Built on x86_64, no regressions found.

gdb/ChangeLog:
2017-04-12  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (HFILES_NO_SRCDIR): Add "common/gdb_termios.h".
	* common/common.m4: Check headers 'termios.h', 'termio.h' and
	'sgtty.h'.
	* common/gdb_termios.h: New file, with parts of "terminal.h".
	* inflow.c: Include "gdb_termios.h".
	* ser-unix.c: Include "gdb_termios.h".
	* terminal.h: Move terminal-related defines to
	"common/gdb_termios.h".

gdb/gdbserver/ChangeLog:
2017-04-12  Sergio Durigan Junior  <sergiodj@redhat.com>

	* remote-utils.c: Include "gdb_termios.h" instead of
	"terminal.h".
	* terminal.h: Delete file.

gdb/ser-unix.c

@@ -31,6 +31,7 @@
 #include "gdb_select.h"
 #include "gdbcmd.h"
 #include "filestuff.h"
+#include "gdb_termios.h"
 
 #ifdef HAVE_TERMIOS