Add new @-code: PROT, user's default file transfer protocol (character)

This fixes issue #629

src/sbbs3/atcodes.cpp

@@ -651,6 +651,10 @@ const char* sbbs_t::atcode(char* sp, char* str, size_t maxlen, int* pmode, bool
 	if(strcmp(sp, "TMP") == 0)
 		return useron.tmpext;
 
+	if(strcmp(sp, "PROT") == 0) {
+		safe_snprintf(str, maxlen, "%c", useron.prot);
+		return str;
+	}
 	if(strcmp(sp, "SEX") == 0) {
 		safe_snprintf(str, maxlen, "%c", useron.sex);
 		return str;