Follow-up to r1591919: Fix fs tests on Windows and non-threaded platforms
by always using our svn_mutex__t and thus being able to detect recursive
locking attempts.

For non-threaded platforms, always using svn_mutex__t adds almost zero
overhead to the FS file locking that we need to do anyways.  On Windows,
the overhead slightly larger but all the APR functions that svn_mutex__*
calls have very efficient implementations on Windows.  So here, the relative
overhead is minimal as well.

OTOH, all our tested platforms will detect incorrect / non-portable lock
usage.

* subversion/libsvn_fs_fs/fs.h
  (SVN_FS_FS__USE_LOCK_MUTEX): Drop. 

* subversion/libsvn_fs_fs/fs.c
  (fs_serialized_init): We always use our mutex structure.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1593015 13f79535-47bb-0310-9956-ffa450edef68