Preserve the real value of -jN in MAKEFLAGS using jobserver.

Previously if the jobserver was active, MAKEFLAGS would contain only
the -j option but not the number (not -j5 or whatever) so users
could not discover that value.  Allow that value to be provided in
MAKEFLAGS without error but still give warnings if -jN is provided
on the command line if the jobserver is already activated.

* NEWS: Discuss the new behavior.
* os.h, posixos.c, w32/w32os.c: Return success/failure from
jobserver_setup() and jobserver_parse_auth().
* main.c (main): Separate the command line storage of job slots (now
in arg_job_slots) from the control storage (in job_slots).  Make a
distinction between -jN flags read from MAKEFLAGS and those seen
on the command line: for the latter if the jobserver is enabled then
warn and disable it, as before.
* tests/scripts/features/jobserver: Add new testing.
9 files changed