Fix some serious, hard-core breakage :-( I don't even know how it

Fix some serious, hard-core breakage :-(  I don't even know how it
compiled and linked.

* there is no UNIX symbol, so go back to #ifndef WIN32. that other
  platforms can pile on as needed until we have a proper mechanism.

* reinstate the liveprop callbacks and the dav_hooks_liveprop_fs
  structure. since the latter structure is referred to elsewhere in
  the file, it better be defined.

* use a "feature" name of DAV_FS_HAS_EXECUTABLE rather than
  conditionally specifying the property. convert the tests over to
  that symbol.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93012 13f79535-47bb-0310-9956-ffa450edef68