PHP mule support

- add PHP functions: 
 - `uwsgi_mule_msg()`
 - `uwsgi_mule_get_msg($manage_signals=true, $manage_farms=true, $timeout=-1, $buffer_size=65536)`
- add `php-mule-hook` option
 - specified script will be executed via `uwsgi_mule_handler()`
 - mule mssage is passed as `$_SERVER['UWSGI_MULE_MSG']`
- add programmed mule support
 - i.e. `uwsgi --mule=mule.php`