Solve a bug identified by Sebastian that we would always reaquire the
  lock immediately instead of stalling for the isapi module's thread
  to complete its work.  This requires the new changes to thread_mutex
  that introduce a guarenteed APR_THREAD_MUTEX_UNNESTED flag.

Reported by: Sebastian Hantsch <Sebastian.Hantsch@t-online.de>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95366 13f79535-47bb-0310-9956-ffa450edef68