libxl: event system: properly register the SIGCHLD self-pipe

libxl: event system: properly register the SIGCHLD self-pipe

An application which uses libxl_osevent_register_hooks, and doesn't
use libxl_sigchld_owner_mainloop, would never properly experience the
deaths of its (libxl) children.

This is because the SIGCHLD self pipe would be handled using ad-hoc
code in beforepoll_internal and afterpoll_internal.  However, this is
no good if application is using the fd registration system instead; in
that case these functions would not be called and nothing would deal
with readability of the self pipe.

Fix this as follows:

The SIGCHLD self pipe now is now dealt with by a standard libxl__ev_fd
handler, which is registered and deregistered along with the SIGCHLD
handler itself.  The handler function subsumes the ad-hoc response
code removed from beforepoll_internal, and the functionality of
libxl__fork_selfpipe_woken.

This is also tidier as the SIGCHLD self pipe is no longer touched
outside libxl_fork.c other than in ctx initialisation and teardown.

(The ad-hoc arrangements for poller->wakeup_pipe in
beforepoll_internal and afterpoll_internal are OK because the
libxl__poller mechanism exists to wake up threads which are sitting
inside libxl's poll loop, so is not applicable to the application's
event loop.)

Reported-by: Bamvor Jian Zhang <bjzhang@suse.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Bamvor Jian Zhang <bjzhang@suse.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>