[SV 66499] Detect jobserver values that are too large

[SV 66499] Detect jobserver values that are too large

Set the jobserver pipe to non-blocking before writing tokens; if a
token write fails the user's jobserver value is too large so fail.

Original implementation: Dmitry Goncharov <dgoncharov@users.sf.net>

* src/posixos.c (set_blocking): Split into force_blocking() which
always enables/disables blocking; set_blocking() may call it.
(jobserver_setup): Set the write side of the pipe to non-blocking
before writing tokens.  If it fails with EAGAIN we know the pipe
is full: create a fatal error.
* tests/scripts/features/jobserver: Test a too-large jobserver.