Add #ifndef MONGOOSE_NO_FILESYSTEM

added a new handler callback to be used when the http connection is being closed

so that in case of connections closed prematurely, the user can clear resources
eventually bound to the connection using the 'connection_param' member of the
mg_connection structure

use `mg_set_request_handler`

Correct return values: PROCESSED or CALL_AGAIN

bugfix: set the correct status if we still have pending data