mod_dav_fs properties using Extended Attributes

Removes the requirement for .DAV metadata directories and a dbm database per file with properties.

Patch for httpd2.3.0-dev:
- http://privsep.org/patches/2.3.0-dev/xattr-patches/httpd-dav-fs-xattrs.patch
Requires AprExtendedAttributes patch (implemented for and tested on Mac OS X, Linux, FreeBSD and Solaris)
- http://issues.apache.org/bugzilla/show_bug.cgi?id=44127
- http://privsep.org/patches/2.3.0-dev/xattr-patches/apr-xattr-all.patch
Adds new global resource conf directive: DAVPropDBType. Allowed values are "sdbm" (default) or "xattr"

  # Enabled mod_dav_fs extended attributes
  DAVPropDBType "xattr"

Does not change default behaviour of mod_dav_fs (sdbm is enabled by default and extended attribute support must be explicitly enabled).
- dbm.c is split into 2 files by the patch:
  - props.c - props provider and namespace handling
  - dbm.c - db specific functions
- No changes are made to dbm functions other than adding a function dispatch structure to call the existing dbm wrapper routines.
- xattr.c implements the dispatch routines and instead stores as extended attributes. The dispatch is switched globally by the config directive.
Creates attributes with exactly the same namespace storage mapping and metadata as the existing SDBM properties.
Extended attribute property keys are prefixed with apache.org:mod_dav_fs: when stored as extended attributes
- This is so we do not clash with existing extended attributes.
- It may be desirabe later to add directives to map certain namespaces+key combinations to non-prefixed extended attributes to allow outside applications easier access to Extended Attributes set by mod_dav_fs.
Passes all litmus properties tests (more testing required).
Example Extended Attributes set during a litmus run (using apxattr sample utility attached to the APR issue link above):

# apxattr -l /opt/apache2.3-davxattrs/uploads/litmus/prop2 
apache.org:mod_dav_fs:1:prop5
apache.org:mod_dav_fs:1:prop6
apache.org:mod_dav_fs:1:prop7
apache.org:mod_dav_fs:1:prop8
apache.org:mod_dav_fs:1:prop9
apache.org:mod_dav_fs:METADATA
apache.org:mod_dav_fs:1:valnspace
apache.org:mod_dav_fs::nonamespace
apache.org:mod_dav_fs:1:high-unicode

To Do
- Documentation changes
- Review
- More testing
- ...