pgsql: Fix canAcceptConnections() bugs introduced by replication-relate

Fix canAcceptConnections() bugs introduced by replication-related patches.

We must not return any "okay to proceed" result code without having checked
for too many children, else we might fail later on when trying to add the
new child to one of the per-child state arrays. It's not clear whether
this oversight explains Stefan Kaltenbrunner's recent report, but it could
certainly produce a similar symptom.

Back-patch to 8.4; the logic was not broken before that.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=6318fa0fdb1ad22b489f1f9f8018ffa70e2e724c

Modified Files
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src/backend/postmaster/postmaster.c | 29 +++++++++++++++++------------
1 files changed, 17 insertions(+), 12 deletions(-)