Add a mod_dav_svn handler to allow the server admin to GET the FSFS
global cache statistics.

* subversion/mod_dav_svn/dav_svn.h
  (dav_svn__status): New.

* subversion/mod_dav_svn/mod_dav_svn.c
  (register_hooks): Register new hook.

* subversion/mod_dav_svn/status.c: New file.

* build/ac-macros/apache.m4: Check for unistd.h and getpid().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1595160 13f79535-47bb-0310-9956-ffa450edef68