Unlike the I-pipe, Dovetail does not copy the work descriptor but
merely hands over the request to the common irq_work() mechanism. We
must guarantee that such descriptor lives in a portion of memory which
won't go stale until the handler has run, which by design can only
happen once the calling out-of-band context unwinds.

Therefore, we have to create signal slots per possible cause in the
thread's control block in order to overcome sigwork in xnthread_signal.

For SIGDEBUG, we are only interested in the very first event coming in,
so one slot is enough. All SIGSHADOW_* events need their own slot:
SIGSHADOW_ACTION_HARDEN and SIGSHADOW_ACTION_HOME can be raised by
remote threads asynchronously to the target thread.
SIGSHADOW_ACTION_BACKTRACE comes in addition to SIGDEBUG_MIGRATE_*. For
the latter reason, SIGSHADOW_ACTION_BACKTRACE cannot pile up though.

Including SIGTERM, we have totally 5 slots.

To ensure that multiple asynchronous signals on the same slot do not
overwrite each other (e.g. SIGSHADOW_ACTION_HOME or SIGDEBUG reasons),
synchronize the slot usage under nklock. Create a __xnthread_signal
variant that can be called from already locked contexts (the majority of
xnthread_signal users).

Based on original patch by Hongzhan Chen.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>