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 <1250860954.1239.114.camel@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.

Tags:
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REL8_4_STABLE

Modified Files:
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    pgsql/src/backend/postmaster:
        autovacuum.c (r1.99 -> r1.99.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.99&r2=1.99.2.1)
        postmaster.c (r1.583.2.3 -> r1.583.2.4)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c?r1=1.583.2.3&r2=1.583.2.4)