Add runas_check_shell flag to require a runas user to have a valid shell.

Not enabled by default.

plugins/sudoers/def_data.c

@@ -529,6 +529,10 @@ struct sudo_defs_types sudo_defs_table[] = {
 	"runas_allow_unknown_id", T_FLAG,
 	N_("Allow the use of unknown runas user and/or group ID"),
 	NULL,
+    }, {
+	"runas_check_shell", T_FLAG,
+	N_("Only permit running commands as a user with a valid shell"),
+	NULL,
     }, {
 	NULL, 0, NULL
     }