Rely on gnulib's unistd.h replacement.

Rely on gnulib's unistd.h replacement.

With gnulib's unistd module, we can assume unistd.h is always present, and that
STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO are always defined.

Don't remove unistd.h from GDB's configure.ac, as later tests in the
file use HAVE_UNISTD_H checks.

gdb/
2013-07-01  Pedro Alves  <palves@redhat.com>

	* defs.h: Don't check HAVE_UNISTD_H before including <unistd.h>.
	(STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO): Delete.
	* tracepoint.c: Don't check HAVE_UNISTD_H before including
	<unistd.h>.

gdb/gdbserver/
2013-07-01  Pedro Alves  <palves@redhat.com>

	* event-loop.c: Don't check HAVE_UNISTD_H before including
	<unistd.h>.
	* gdbreplay.c: Likewise.
	* remote-utils.c: Likewise.
	* server.c: Likewise.
	* configure.ac: Don't check for unistd.h.
	* configure: Regenerate.