pgsql: Fix crash caused by log_timezone patch if we attempt to emit any

Log Message:
-----------
Fix crash caused by log_timezone patch if we attempt to emit any elog messages
between the setting of log_line_prefix and the setting of log_timezone.  We
can't realistically set log_timezone any earlier than we do now, so the best
behavior seems to be to use GMT zone if any timestamps are to be logged during
early startup.  Create a dummy zone variable with a minimal definition of GMT
(in particular it will never know about leap seconds), so that we can set it
up without reference to any external files.

Modified Files:
--------------
    pgsql/src/backend/utils/error:
        elog.c (r1.192 -> r1.193)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c.diff?r1=1.192&r2=1.193)
    pgsql/src/backend/utils/misc:
        guc.c (r1.409 -> r1.410)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.409&r2=1.410)
    pgsql/src/include:
        pgtime.h (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgtime.h.diff?r1=1.16&r2=1.17)
    pgsql/src/timezone:
        localtime.c (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/timezone/localtime.c.diff?r1=1.16&r2=1.17)
        pgtz.c (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/timezone/pgtz.c.diff?r1=1.52&r2=1.53)