Fix Y2038 issues with MyStartTime.

Several places treat MyStartTime as a "long", which is only 32 bits
wide on some platforms.  In reality, MyStartTime is a pg_time_t,
i.e., a signed 64-bit integer.  This will lead to interesting bugs
on the aforementioned systems in 2038 when signed 32-bit integers
are no longer sufficient to store Unix time (e.g., "pg_ctl start"
hanging).  To fix, ensure that MyStartTime is handled as a 64-bit
value everywhere.  (Of course, users will need to ensure that
time_t is 64 bits wide on their system, too.)

Co-authored-by: Max Johnson
Discussion: https://postgr.es/m/CO1PR07MB905262E8AC270FAAACED66008D682%40CO1PR07MB9052.namprd07.prod.outlook.com
Backpatch-through: 12

src/backend/utils/init/miscinit.c

@@ -1372,10 +1372,10 @@ CreateLockFile(const char *filename, bool amPostmaster,
 	 * both datadir and socket lockfiles; although more stuff may get added to
 	 * the datadir lockfile later.
 	 */
-	snprintf(buffer, sizeof(buffer), "%d\n%s\n%ld\n%d\n%s\n",
+	snprintf(buffer, sizeof(buffer), "%d\n%s\n" INT64_FORMAT "\n%d\n%s\n",
 			 amPostmaster ? (int) my_pid : -((int) my_pid),
 			 DataDir,
-			 (long) MyStartTime,
+			 MyStartTime,
 			 PostPortNumber,
 			 socketDir);