[manual] Job control is no longer optional.

Job control was made mandatory in POSIX.1-2001: compare
<http://pubs.opengroup.org/onlinepubs/7990989775/xsh/unistd.h.html> with
<http://pubs.opengroup.org/onlinepubs/009695399/basedefs/unistd.h.html>.
Seventeen years later, we need not devote an entire manual @node to
warning people that this was once an optional POSIX feature.

	* manual/job.texi (Job Control is Optional): Remove node, as
	job control has not been optional in quite some time.
	(Job Control): Mention briefly that systems older than
	POSIX.1-2001 might not support job control.
	* manual/conf.texi (_POSIX_JOB_CONTROL): Will always be
	defined on systems conforming to POSIX.1-2001.