Give mod_dav the ability to output both standard and customized

Give mod_dav the ability to output both standard and customized
<D:error> responses.  It's crucial for marshalling svn error messages
back over to the client; and someday it will be needed to return
specific <DAV:> errors as dictated by the DeltaV spec.

* mod_dav.h (dav_error): add two new fields -- an optional error
    namespace, and an error-tag-name.  Remove the 'delayed computation'
    function and cxt ptrs in this struct;  they were never used.

  (dav_new_error_tag):  new alternative constructor that takes new
    fields.

* util.c (dav_new_error_tag):  implement constructor.

* mod_dav.c (dav_error_response_tag):  new function to output
    'standard' xml error response based on error struct.

  (dav_handle_err): if no multistatus response is passed in, and if an
    error-tag is defined, then call our new xml-output routine.

Submitted by: Ben Collins-Sussman <sussman@collab.net>

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93651 13f79535-47bb-0310-9956-ffa450edef68