Fix @UPTIME@ to return 0 if the uptime global isn't initialized yet, or

if time_t is unsigned and the current time is less than uptime.

There's comments that suggest the time(NULL) value is influenced by the
timezone, so possibly therre's a usage of it somewhere with the timezone
not being set comparing to an uptime that was set when timezone *was* set.
In this case, @UPTIME@ will say it's been up for zero days until the timezone
is set in that thread/process, or time(NULL) passes uptime.

src/sbbs3/atcodes.cpp

@@ -147,9 +147,10 @@ const char* sbbs_t::atcode(char* sp, char* str, size_t maxlen)
 
 	if(!strcmp(sp,"UPTIME")) {
 		extern volatile time_t uptime;
-		time_t up=time(NULL)-uptime;
-		if(up<0)
-			up=0;
+		time_t up=0;
+		now = time(NULL);
+		if (uptime != 0 && now >= uptime)
+			up = now-uptime;
 		char   days[64]="";
 		if((up/(24*60*60))>=2) {
 	        sprintf(days,"%lu days ",(ulong)(up/(24L*60L*60L)));