* src/posixos.c (job_root): Remember root jobserver instances

* src/posixos.c (job_root): Remember root jobserver instances

(jobserver_setup): Set job_root to true.
(jobserver_clear): if we're the root instance and we're using a
FIFO, unlink it.  If we're not in a signal handler, free memory.
(jobserver_acquire_all): Call jobserver_clear().
(sync_root): Rename from sync_parent for consistency.