pgsql: Prevent starting a standalone backend with standby_mode on.

Prevent starting a standalone backend with standby_mode on.

This can't really work because standby_mode expects there to be more
WAL arriving, which there will not ever be because there's no WAL
receiver process to fetch it.  Moreover, if standby_mode is on then
hot standby might also be turned on, causing even more strangeness
because that expects read-only sessions to be executing in parallel.
Bernd Helmle reported a case where btree_xlog_delete_get_latestRemovedXid
got confused, but rather than band-aiding individual problems it seems
best to prevent getting anywhere near this state in the first place.
Back-patch to all supported branches.

In passing, also fix some omissions of errcodes in other ereport's in
readRecoveryCommandFile().

Michael Paquier (errcode hacking by me)

Discussion: <00F0B2CEF6D0CEF8A90119D4@eje.credativ.lan>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/c40bb11559c516c24ff1460e2fac5da20abfeb39

Modified Files
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src/backend/access/transam/xlog.c | 24 +++++++++++++++++++-----
1 file changed, 19 insertions(+), 5 deletions(-)