consolidate logic to remove the pidfile in a new API,

ap_remove_pid(), related to existing ap_log_pid() and
ap_read_pid()

presumably this is useful to third-party MPMs as well


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1086224 13f79535-47bb-0310-9956-ffa450edef68