On the revprop-cache branch: Since APR proc mutexes are only valid

On the revprop-cache branch: Since APR proc mutexes are only valid
within a process tree, we need our own global mutex mechanism.
It uses a lock file for global sync. and a svn_mutex__t for thread sync.
Also, fix an issue identified by testing without intrinsics.

* subversion/libsvn_subr/svn_named_atomic.c
  (synched_read): must take a reference to the sync'ed data
  (svn_named_atomic__read): adapt caller
  (mutex_pool, mutex, lock_file): data for the new sync mechanism
  (init_mutex, lock, unlock): implement new sync mechanism

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/revprop-cache@1325463 13f79535-47bb-0310-9956-ffa450edef68