pgsql: Add a note to the documentation of pg_standby that it's important

Log Message:
-----------
Add a note to the documentation of pg_standby that it's important that
the postgres process has permissions to delete the trigger file, per
suggestion by Mason Hale.

Also fix pg_standby to do a more predictable exit(200) instead of the
current exit(-1) when the unlink of the trigger file fails anyway.

This only affects 8.3 branch. Older versions didn't have pg_standby,
and in 8.4 upwards pg_standby is no longer responsible for deleting the
trigger file; it's supposed to be done by recovery_end_command instead.

Tags:
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REL8_3_STABLE

Modified Files:
--------------
pgsql/contrib/pg_standby:
pg_standby.c (r1.10.2.7 -> r1.10.2.8)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_standby/pg_standby.c?r1=1.10.2.7&r2=1.10.2.8)
pgsql/doc/src/sgml:
pgstandby.sgml (r2.4.2.1 -> r2.4.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pgstandby.sgml?r1=2.4.2.1&r2=2.4.2.2)