dest.c: Use monotonic time for cups_enum_dest

Patch written by Adam Williamson.

`gettimeofday()` is vulnerable to clock jumps, so it is better to use
`clock_gettime()` to avoid issues with libcups if system changes clock,
f.e. in Live CDs.

cups/dest.c

@@ -222,7 +222,7 @@ static const char	*cups_dnssd_resolve(cups_dest_t *dest, const char *uri,
 static int		cups_dnssd_resolve_cb(void *context);
 static void		cups_dnssd_unquote(char *dst, const char *src,
 			                   size_t dstsize);
-static int		cups_elapsed(struct timeval *t);
+static int		cups_elapsed(struct timespec *t);
 #endif /* HAVE_DNSSD */
 static int              cups_enum_dests(http_t *http, unsigned flags, int msec, int *cancel, cups_ptype_t type, cups_ptype_t mask, cups_dest_cb_t cb, void *user_data);
 static int		cups_find_dest(const char *name, const char *instance,
@@ -3404,15 +3404,14 @@ cups_dnssd_unquote(char       *dst,	/* I - Destination buffer */
  */
 
 static int				/* O  - Elapsed time in milliseconds */
-cups_elapsed(struct timeval *t)		/* IO - Previous time */
+cups_elapsed(struct timespec *t)		// IO - Previous time
 {
   int			msecs;		/* Milliseconds */
-  struct timeval	nt;		/* New time */
+  struct timespec	nt;		// New time
 
+  clock_gettime(CLOCK_MONOTONIC, &nt);
 
-  gettimeofday(&nt, NULL);
-
-  msecs = (int)(1000 * (nt.tv_sec - t->tv_sec) + (nt.tv_usec - t->tv_usec) / 1000);
+  msecs = (int)(1000 * (nt.tv_sec - t->tv_sec) + (nt.tv_nsec - t->tv_nsec) / 1000 / 1000);
 
   *t = nt;
 
@@ -3446,7 +3445,7 @@ cups_enum_dests(
   int           count,                  /* Number of queries started */
                 completed,              /* Number of completed queries */
                 remaining;              /* Remainder of timeout */
-  struct timeval curtime;               /* Current time */
+  struct timespec curtime;               // Current time
   _cups_dnssd_data_t data;		/* Data for callback */
   _cups_dnssd_device_t *device;         /* Current device */
 #  ifdef HAVE_MDNSRESPONDER
@@ -3672,7 +3671,7 @@ cups_enum_dests(
   * Get Bonjour-shared printers...
   */
 
-  gettimeofday(&curtime, NULL);
+  clock_gettime(CLOCK_MONOTONIC, &curtime);
 
 #  ifdef HAVE_MDNSRESPONDER
   if (DNSServiceCreateConnection(&data.main_ref) != kDNSServiceErr_NoError)