Allow modules that add sockets to the ap_listeners list to

Allow modules that add sockets to the ap_listeners list to
define the function that should be used to accept on that
socket.  Each MPM can define their own function to use for
the accept function with the MPM_ACCEPT_FUNC macro.  This
also abstracts out all of the Unix accept error handling
logic, which has become out of synch across Unix MPMs.

The code flow is much easier now for different transports:

1)  During pre-config, post-config or while parsing the config
    file, add a socket to the ap_listeners list, making sure to
    define an accept function at the same time.

2)  MPMs find the correct listener, and call the accept function
    that was defined in step 1.

3)  That accept function returns a void pointer, which is passed
    to the create_connection hook.

4)  create_connection adds the correct low-level filters.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91916 13f79535-47bb-0310-9956-ffa450edef68