[SV 51159] Use a non-blocking read with pselect to avoid hangs.

[SV 51159] Use a non-blocking read with pselect to avoid hangs.

* posixos.c (set_blocking): Set blocking on a file descriptor.
(jobserver_setup): Set non-blocking on the jobserver read side.
(jobserver_parse_auth): Ditto.
(jobserver_acquire_all): Set blocking to avoid a busy-wait loop.
(jobserver_acquire): If the non-blocking read() returns without
taking a token then try again.