xenstore,libxl: cleanup of xenstore connections across fork()

xenstore,libxl: cleanup of xenstore connections across fork()

Provide a new function xs_daemon_destroy_postfork which can be called
by a libxenstore user who has called fork, to close the fd for the
connection to xenstored and free the memory, without trying to do
anything to any threads which libxenstore may have created.

Use this new function in libxl_fork, to avoid accidental use of a
xenstore connection in both parent and child.

Also, fix the doc comment for libxl_spawn_spawn to have the success
return codes the right way round.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>