Replace built-in non-unix group support with a sudoers group plugin.

Include a sample plugin that can read Unix-format group files.

plugins/sudoers/def_data.c

@@ -326,6 +326,10 @@ struct sudo_defs_types sudo_defs_table[] = {
 	"use_pty", T_FLAG,
 	"Always run commands in a pseudo-tty",
 	NULL,
+    }, {
+	"group_plugin", T_STR,
+	"Plugin for non-Unix group support",
+	NULL,
     }, {
 	NULL, 0, NULL
     }