[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.