Follow-up to r1591919:  Make recursive lock attempts also detectable
on systems without thread support.  On those, we will simply count
the lock attempts (counter must never exceed 1).  This fixes the
test failures on our BSD build bots.

* subversion/include/private/svn_mutex.h
  (svn_mutex__t): Always have a mutex struct.

* subversion/libsvn_subr/mutex.c
  (svn_mutex__t): On non-threaded systems, the struct contains a
                  simple lock counter.
  (svn_mutex__init): Always allocate and initialize a mutex structure.
  (svn_mutex__lock,
   svn_mutex__unlock): On non-threaded systems, use simple ref counting
                       to detect recursive locking attempts.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1592640 13f79535-47bb-0310-9956-ffa450edef68