Use mkostemp, not mkstemp

I noticed that gdb could leak file descriptors coming from mkstemp.
This patch fixes the problem by importing the gnulib mkostemp instead,
and then changing gdb to pass O_CLOEXEC.

A small gnulib patch was needed.  This has already been accepted
upstream.

gdb/ChangeLog
2018-10-27  Tom Tromey  <tom@tromey.com>

	* unittests/scoped_mmap-selftests.c (test_normal): Use
	gdb_mkostemp_cloexec.
	* unittests/scoped_fd-selftests.c (test_destroy, test_release):
	Use gdb_mkostemp_cloexec.
	* gnulib/aclocal-m4-deps.mk, gnulib/aclocal.m4,
	gnulib/config.in, gnulib/configure,
	gnulib/import/Makefile.am, gnulib/import/Makefile.in,
	gnulib/import/m4/gnulib-cache.m4,
	gnulib/import/m4/gnulib-comp.m4: Update.
	* gnulib/import/m4/mkostemp.m4: New file.
	* gnulib/import/m4/mkstemp.m4: Remove.
	* gnulib/import/mkostemp.c: New file.
	* gnulib/import/mkstemp.m4: Remove.
	* gnulib/update-gnulib.sh (IMPORTED_GNULIB_MODULES): Remove
	mkstemp, add mkostemp.  Apply new patch.
	* gnulib/import/stdlib.in.h: Apply patch.
	* gnulib/patches/0002-mkostemp-mkostemps-Fix-compilation-error-in-C-mode-o.patch:
	New file.
	* dwarf-index-write.c (write_psymtabs_to_index): Use
	gdb_mkostemp_cloexec.
	* common/filestuff.h (gdb_mkostemp_cloexec): New function.