Import the "unistd" gnulib module.

Import the "unistd" gnulib module.

I wanted to import the pathmax module, and that pulls in the unistd
module as dependency.  The unistd module is actually bigger than the
pathmax module.  If we're going to end up with it, might as well
import it explicitly, and make use of it throughout.

The "unistd" module makes a GNU-like <unistd.h> always available.
This means we no longer need to do:

 +#ifdef HAVE_UNISTD_H
  #include <unistd.h>
 +#endif

and we can remove a few constants from defs.h.

This is just the importing step.

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

	Import the "unistd" gnulib module.
	* gnulib/update-gnulib.sh (IMPORTED_GNULIB_MODULES): Add "unistd".
	* gnulib/Makefile.in (aclocal_m4_deps): Add import/m4/off_t.m4,
	import/m4/ssize_t.m4, import/m4/sys_types_h.m4 and
	import/m4/unistd_h.m4.
	* gnulib/aclocal.m4: Renenerate.
	* gnulib/config.in: Renenerate.
	* gnulib/configure: Renenerate.
	* gnulib/import/Makefile.am: Renenerate.
	* gnulib/import/Makefile.in: Renenerate.
	* gnulib/import/m4/gnulib-cache.m4: Renenerate.
	* gnulib/import/m4/gnulib-comp.m4: Renenerate.
	* gnulib/import/m4/off_t.m4: New file.
	* gnulib/import/m4/ssize_t.m4: New file.
	* gnulib/import/m4/sys_types_h.m4: New file.
	* gnulib/import/m4/unistd_h.m4: New file.
	* gnulib/import/sys_types.in.h: New file.
	* gnulib/import/unistd.c: New file.
	* gnulib/import/unistd.in.h: New file.