Make variables in set::reject-message and other buildvarstring() usage

no longer expand shorter versions of a variable. It previously had some
unintended form of magic autocomplete where $serv was handled the same
way as if it was $server. This could cause issues in the long run when
variables are added and the meaning of the short form changes.

Reported by westor in https://bugs.unrealircd.org/view.php?id=6123

src/support.c

@@ -1269,7 +1269,7 @@ void buildvarstring(const char *inbuf, char *outbuf, size_t len, const char *nam
 			/* find variable name in list */
 			found = 0;
 			for (cnt = 0; name[cnt]; cnt++)
-				if (!strncasecmp(name[cnt], i, p - i))
+				if (!strncasecmp(name[cnt], i, strlen(name[cnt])))
 				{
 					/* Found */
 					found = 1;