New @-code: CONDMENU (conditional menu) - displays a menu/display file

only if it exists (on disk) and displays no error message if the file does not
exist. Nearly identical to the existing MENU @-Code, but without the need to
insure the file actually exists first.

src/sbbs3/atcodes.cpp

@@ -647,6 +647,11 @@ const char* sbbs_t::atcode(char* sp, char* str, size_t maxlen)
 		return(nulstr);
 	}
 
+	if(!strncmp(sp,"CONDMENU:",9)) {
+		menu(sp+9, P_NOERROR);
+		return(nulstr);
+	}
+
 	if(!strncmp(sp,"TYPE:",5)) {
 		printfile(cmdstr(sp+5,nulstr,nulstr,str),0);
 		return(nulstr);