Go back to using a callback for runas_default to keep runas_pw in

sync.  This is needed to make per-entry runas_default settings work
with LDAP-based sudoers.  Instead of declaring it a callback in
def_data.in, sudo and testsudoers poke sudo_defs_table[] which is
a bit naughty, but avoids requiring stub functions in visudo and
the tests.

plugins/sudoers/defaults.h

@@ -48,7 +48,7 @@ struct sudo_defs_types {
     int type;
     char *desc;
     struct def_values *values;
-    int (*callback)(char *);
+    int (*callback)(const char *);
     union {
 	int flag;
 	int ival;