Fix a problem found by Ryan when deleting collections, which triggered an

Fix a problem found by Ryan when deleting collections, which triggered an
underlying (broad) bug. dav_add_response() was assuming the walk params were
a dav_walker_ctx. During the walker cleanup in Nov00, that assumption was
removed, so response errors that occurred in the cleaned sections (such as
dav_fs_delete_resource) could trigger a segfault.

Solution: add a pool to dav_walk_resource and alter dav_add_response to use
that, rather than assume the ctx is a dav_walker_ctx.

[ note there is also a pool in dav_walk_resource.resource, but that pool is
  associated with the *resource* rather than the process of walking, so we
  introduced another field. currently they are the same, however. ]

Found by: Ryan Bloom

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87670 13f79535-47bb-0310-9956-ffa450edef68