Restore timezone selection on GPDB startup

Commit b749790a backports changes from Postgres 10.1 to parse the most
recent version of the IANA timezone library format, but these changes
also included some binary incompatible changes in behavior.
Specifically, pg_timezone_pre_initialize() and identify_system_timezone()
were removed, resulting in the timezone defaulting to 'GMT' on database
startup, even if the TimeZone GUC was specifically set in
postgresql.conf.

This commit restores the previous behavior without reverting the IANA
timezone library changes.

src/backend/utils/misc/guc.c

@@ -3362,7 +3362,7 @@ InitializeGUCOptions(void)
 	 * Before log_line_prefix could possibly receive a nonempty setting, make
 	 * sure that timezone processing is minimally alive (see elog.c).
 	 */
-	pg_timezone_initialize();
+	pg_timezone_pre_initialize();
 
 	/*
 	 * Build sorted array of all GUC variables.

src/include/pgtime.h

@@ -76,13 +76,7 @@ extern void pg_tzenumerate_end(pg_tzenum *dir);
 
 extern pg_tz *session_timezone;
 extern pg_tz *log_timezone;
-/*
- * GPDB_92_MERGE_FIXME: Remove when commit ca4af308c32 has been merged.
- * gmt_timezone has been made obsolete with log_timezone being set by
- * pg_timezone_initialize(), but in order to avoid merge conflicts in
- * the code the callers are still allowed to use gmt_timezone.
- */
-#define gmt_timezone log_timezone
+extern pg_tz *gmt_timezone;
 
 /*
  * GPDB_92_MERGE_FIXME: Remove when commit ca4af308c32 is merged. This