Bug #59017: Auth plugin socket_peercred fails to load in 5.5.8

Due to how the server checks for option names the name of the
plugin "socket_peercred" collides with the existing server side 
option "socket".

Renamed the plugin from "socket_peercred" to "auth_socket" to
avoid the name clash.

plugin/auth/auth_socket.c

@@ -79,7 +79,7 @@ mysql_declare_plugin(socket_auth)
 {
   MYSQL_AUTHENTICATION_PLUGIN,
   &socket_auth_handler,
-  "socket_peercred",
+  "auth_socket",
   "Sergei Golubchik",
   "Unix Socket based authentication",
   PLUGIN_LICENSE_GPL,