Allow mod_dav_fs to tolerate race conditions between PROPFIND and an

Allow mod_dav_fs to tolerate race conditions between PROPFIND and an
operation which removes a directory/file between apr_dir_read() and
apr_stat(). Current behaviour is to abort the connection which seems
inferior to tolerating (and logging) the error.

* modules/dav/fs/repos.c (dav_fs_walker): If DAV_WALKTYPE_TOLERANT is
  set, ignore ENOENT from stat() rather than aborting the response.
  Log the error from stat either way.

* modules/dav/main/mod_dav.c (dav_method_propfind): Set
  DAV_WALKTYPE_TOLERANT.

* modules/dav/main/mod_dav.h: Add DAV_WALKTYPE_TOLERANT.

Github: closes #369

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1910811 13f79535-47bb-0310-9956-ffa450edef68