hwclock: fix signed/unsigned comparison warning on alpha

Fixes the following warning:
sys-utils/hwclock.c: In function 'manipulate_epoch':
sys-utils/hwclock.c:1465:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (epoch_opt == -1)

Full build log available on:
https://buildd.debian.org/status/fetch.php?pkg=util-linux&arch=alpha&ver=2.26.2-9&stamp=1440078034

Detected by/via:
https://qa.debian.org/bls/packages/u/util-linux.html

Please note that this has never actually been (build-)tested on alpha,
but should hopefully resolve the warning. Note also that limits.h is
already included.

Signed-off-by: Andreas Henriksson <andreas@fatal.se>

sys-utils/hwclock.c

@@ -137,7 +137,7 @@ int debug;
 bool badyear;
 
 /* User-specified epoch, used when rtc fails to return epoch. */
-unsigned long epoch_option = -1;
+unsigned long epoch_option = ULONG_MAX;
 
 /*
  * Almost all Award BIOS's made between 04/26/94 and 05/31/95 have a nasty
@@ -1452,7 +1452,7 @@ manipulate_epoch(const bool getepoch,
 			printf(_("Kernel is assuming an epoch value of %lu\n"),
 			       epoch);
 	} else if (setepoch) {
-		if (epoch_opt == -1)
+		if (epoch_opt == ULONG_MAX)
 			warnx(_
 			      ("To set the epoch value, you must use the 'epoch' "
 			       "option to tell to what value to set it."));