libxl: fork: Provide ..._always_selective_reap

libxl: fork: Provide ..._always_selective_reap

Applications exist which want to use libxl in an event-driven mode but
which do not integrate child termination into their event system, but
instead reap all their own children synchronously.

In such an application libxl must own SIGCHLD but avoid reaping any
children that don't belong to libxl.

Provide libxl_sigchld_owner_libxl_always_selective_reap which has this
behaviour.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
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v2: Document the new mode in the big "Subprocess handling" comment.