Add -t option to htcacheclean to clean out empty directories.

* support/htcacheclean.c: Implement -t option.
* docs/man/htcacheclean.8, docs/manual/programs/htcacheclean.html.en,
  docs/manual/programs/htcacheclean.xml: Document -t option.

Justin did some minor tweaks as he agrees with Andreas and Rudiger that
apr_dir_remove should return an error if the directory isn't empty; hence,
remove the apr_dir_read calls.

Submitted by: Colm MacCarthaigh
Reviewed by:  Justin Erenkrantz, Andreas Steinmetz, Rudiger Pluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@231351 13f79535-47bb-0310-9956-ffa450edef68