Revamp how mod_dav asks its provider to generate a GET response.

* eliminate the get_pathname and free_file vtable functions. add the
    deliver() function to have the provider deliver the content
    straight into a filter [stack].

* eliminate readable streams -- they are now obsolete. this gets rid
    of the read_stream vtable function and DAV_MODE_READ*

* implement a deliver() function for the FS provider. this simply
    constructs a bucket and EOS and shoves them into the filter. note
    that this is debug code only, so the "large file" issue handled by
    the core's default handler doesn't count here.

* allow the provider to handle GET for any resource type and for
    collections. this moves the checks into the provider.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92848 13f79535-47bb-0310-9956-ffa450edef68