sslutils: expire time in local timezone format

Just turning attached patch of github issue #840 into a push request.

THANKS.in

@@ -309,3 +309,4 @@ Tilmann Bubeck
 Stéphane Bortzmeyer
 Luca Corti
 Jethro Carr
+Neil Prockter

plugins/sslutils.c

@@ -151,7 +151,8 @@ int np_net_ssl_check_cert(int days_till_exp_warn, int days_till_exp_crit){
 	struct tm stamp;
 	float time_left;
 	int days_left;
-	char timestamp[17] = "";
+	char timestamp[50] = "";
+	time_t tm_t;
 
 	certificate=SSL_get_peer_certificate(s);
 	if (!certificate) {
@@ -209,10 +210,8 @@ int np_net_ssl_check_cert(int days_till_exp_warn, int days_till_exp_crit){
 
 	time_left = difftime(timegm(&stamp), time(NULL));
 	days_left = time_left / 86400;
-	snprintf
-		(timestamp, 17, "%02d/%02d/%04d %02d:%02d",
-		 stamp.tm_mon + 1,
-		 stamp.tm_mday, stamp.tm_year + 1900, stamp.tm_hour, stamp.tm_min);
+	tm_t = mktime (&stamp);
+	strftime(timestamp, 50, "%c", localtime(&tm_t));
 
 	if (days_left > 0 && days_left <= days_till_exp_warn) {
 		printf (_("%s - Certificate '%s' expires in %d day(s) (%s).\n"), (days_left>days_till_exp_crit)?"WARNING":"CRITICAL", cn, days_left, timestamp);