BUG/MEDIUM: threads: unbreak "bind" referencing an incorrect thread number

BUG/MEDIUM: threads: unbreak "bind" referencing an incorrect thread number

The "process" directive on "bind" lines supports process references and
thread references. No check is performed on the thread number validity,
so that if a listener is only bound to non-existent threads, the traffic
will never be processed. It easily happens when setting one bind line per
thread with an incorrect (or reduced) thread count. No warning appears
and some random connections are never served. It also happens when setting
thread references with threads support disabled at build time.

This patch makes use of the all_threads_mask variable to detect if some
referenced threads don't exist, to emit a warning and fix this.

This patch needs to be backported to 1.8, just like the previous one which
it depends on (MINOR: threads: move "nbthread" parsing to hathreads.c).