pgsql: Avoid calling kill() in a postmaster signal handler.

Log Message:
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Avoid calling kill() in a postmaster signal handler.

This causes problems when the system load is high, per report from Zdenek
Kotala in <1250860954(dot)1239(dot)114(dot)camel(at)localhost>; instead of calling kill
directly, have the signal handler set a flag which is checked in ServerLoop.
This way, the handler can return before being called again by a subsequent
signal sent from the autovacuum launcher. Also, increase the sleep in the
launcher in this failure path to 1 second.

Backpatch to 8.3, which is when the signalling between autovacuum
launcher/postmaster was introduced.

Also, add a couple of ReleasePostmasterChildSlot calls in error paths; this
part backpatched to 8.4 which is when the child slot stuff was introduced.

Modified Files:
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pgsql/src/backend/postmaster:
autovacuum.c (r1.101 -> r1.102)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.101&r2=1.102)
postmaster.c (r1.587 -> r1.588)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c?r1=1.587&r2=1.588)