Add admin_flag sudoers option and make --enable-admin-flag take a path.

It is now possible to disable the Ubuntu admin flag in sudoers
or change its location.
GitHub issue #56

plugins/sudoers/def_data.c

@@ -573,6 +573,10 @@ struct sudo_defs_types sudo_defs_table[] = {
 	"selinux", T_FLAG,
 	N_("Enable SELinux RBAC support"),
 	NULL,
+    }, {
+	"admin_flag", T_STR|T_BOOL|T_CHPATH,
+	N_("Path to the file that is created the first time sudo is run: %s"),
+	NULL,
     }, {
 	NULL, 0, NULL
     }