New @-code NODE_USER which will expand to UNKNOWN_USER if node is anonymous

Needed for exactly reproducing built-in node status string formatting using
@-codes (only).

src/sbbs3/atcodes.cpp

@@ -957,6 +957,9 @@ const char* sbbs_t::atcode(const char* sp, char* str, size_t maxlen, int* pmode,
 	if(!strcmp(sp,"USER") || !strcmp(sp,"ALIAS") || !strcmp(sp,"NAME"))
 		return(useron.alias);
 
+	if(strcmp(sp, "NODE_USER") == 0)
+		return thisnode.misc&NODE_ANON ? text[UNKNOWN_USER] : useron.alias;
+
 	if(!strcmp(sp,"FIRST")) {
 		safe_snprintf(str,maxlen,"%s",useron.alias);
 		tp=strchr(str,' ');