System cell: fix kill command

The patch http://rb.dcache.org/r/4278/, committed to master@7071473dc27df13
(pre-2.2) and merged 1.9.12@51e03907399a510 fixes the problem that a
kill command may take time to complete. In particular, when working
around a configuration issue, an existing gPlazma is killed before a
fresh gPlazma is started. If the new gPlazma is started before the old
cell has died then the new gPlazma will not start. This is the
race-condition the patch fixes.

Unfortunately, the patch also introduces a deadlock. When attempting
to kill the System cell, the message-processing thread is blocked
waiting for a signal that the cell has died. This signal is generated
when processing an event by the message-processing thread which,
because the kill command is blocking, will never happen.

This patch fixes the problem by replying with a DelayedReply; an
implementation of the future Reply. The patch also adds support for
replying with a Reply to SystemCell.

Target: trunk
Request: 2.2
Request: 1.9.12
Require-notes: yes
Require-book: no
Patch: http://rb.dcache.org/r/5115/
Acked-by: Gerd Behrmann
Committed: master@d20662a349d6a520fa1dd3b5dba61d7b905b9d97