Remove special-case treatment of LOG severity level in standalone mode.

elog.c has historically treated LOG messages as low-priority during
bootstrap and standalone operation.  This has led to confusion and even
masked a bug, because the normal expectation of code authors is that
elog(LOG) will put something into the postmaster log, and that wasn't
happening during initdb.  So get rid of the special-case rule and make
the priority order the same as it is in normal operation.  To keep from
cluttering initdb's output and the behavior of a standalone backend,
tweak the severity level of three messages routinely issued by xlog.c
during startup and shutdown so that they won't appear in these cases.
Per my proposal back in December.