mpm_event: follow up to r1818804.

Allow DONE as a successful ap_run_process_connection() return value, for
instance h2_conn_run() and h2_task_process_conn() uses it, third-party
modules may too...



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1818960 13f79535-47bb-0310-9956-ffa450edef68