support: Add support_capture_subprogram

support: Add support_capture_subprogram

Its API is similar to support_capture_subprocess, but rather creates a
new process based on the input path and arguments.  Under the hoods it
uses posix_spawn to create the new process.

It also allows the use of other support_capture_* functions to check
for expected results and free the resources.

Checked on x86_64-linux-gnu.

	* support/Makefile (libsupport-routines): Add support_subprocess,
	xposix_spawn, xposix_spawn_file_actions_addclose, and
	xposix_spawn_file_actions_adddup2.
	(tst-support_capture_subprocess-ARGS): New rule.
	* support/capture_subprocess.h (support_capture_subprogram): New
	prototype.
	* support/support_capture_subprocess.c (support_capture_subprocess):
	Refactor to use support_subprocess and support_capture_poll.
	(support_capture_subprogram): New function.
	* support/tst-support_capture_subprocess.c (write_mode_to_str,
	str_to_write_mode, test_common, parse_int, handle_restart,
	do_subprocess, do_subprogram, do_multiple_tests): New functions.
	(do_test): Add support_capture_subprogram tests.
	* support/subprocess.h: New file.
	* support/support_subprocess.c: Likewise.
	* support/xposix_spawn.c: Likewise.
	* support/xposix_spawn_file_actions_addclose.c: Likewise.
	* support/xposix_spawn_file_actions_adddup2.c: Likewise.
	* support/xspawn.h: Likewise.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>