Revamp the API that mod_dav uses to talk to back end deadprop (propdb)

Revamp the API that mod_dav uses to talk to back end deadprop (propdb)
providers. The old API was really based on how the FS stored properties, but
sucked for other types of providers (SQL databases, Subversion, etc). This
new code is overall much cleaner as it moves from a DBM style API to one
tuned for actual mod_dav operation; it also more flexible/clearer for future
improvements (e.g. dropping props directly to the wire rather than buffering
in memory).

This new API allows the provider to better define namespace handling during
the output of the values, how values are serialized and stored, the mapping
between elements' namespaces and the internalized namespace storage, a
clearer mechanism for naming properties (dav_prop_name), and an explicit
rollback mechanism to deal with PROPPATCH atomicity.

Updated the FS provider (fs/dbm.c) to the new API, mostly by moving code
from main/props.c. Of course, with the new semantics, some big changes in
the namespace mapping were made.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91065 13f79535-47bb-0310-9956-ffa450edef68