libxl: event system: Make libxl_sigchld_owner_libxl_always work

libxl: event system: Make libxl_sigchld_owner_libxl_always work

Previously, libxl_sigchld_owner_libxl_always was mishandled.  It would
result in libxl paying no attention to the sigchld self pipe.

Fix this by fixing chldmode_ours so that it returns true iff we are
supposed to be handling SIGCHLD.

Additionally, we arrange to use chldmode_ours everywhere where we are
installing/removing signal handlers and/or deciding whether to check
the self pipe, etc.  This means it needs a new "creating" flag
argument for the benefit of libxl__ev_child_fork, which needs to
install the signal handler in libxl_sigchld_owner_libxl even if there
are not currently any children.

ctx->childproc_hooks->chldowner is now interpreted only by
chldmode_ours.

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>
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v2: Get sense of chldmode test right in perhaps_removehandler (!)