pgsql: Code review for recent patch to terminate online backup during

Log Message:
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Code review for recent patch to terminate online backup during shutdown:
do CancelBackup at a sane place, fix some oversights in the state transitions,
allow only superusers to connect while we are waiting for backup mode to end.

Modified Files:
--------------
pgsql/doc/src/sgml:
runtime.sgml (r1.415 -> r1.416)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml?r1=1.415&r2=1.416)
pgsql/src/backend/postmaster:
postmaster.c (r1.555 -> r1.556)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c?r1=1.555&r2=1.556)
pgsql/src/backend/utils/init:
postinit.c (r1.182 -> r1.183)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c?r1=1.182&r2=1.183)
pgsql/src/include/libpq:
libpq-be.h (r1.65 -> r1.66)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq-be.h?r1=1.65&r2=1.66)