Merge of r4933, r4933: shared memory fixes.

Merge of r4933, r4933: shared memory fixes.

*) Fixed location of debug message in ngx_shmtx_lock().

*) Core: don't reuse shared memory zone that changed ownership (ticket #210).

   nginx doesn't allow the same shared memory zone to be used for different
   purposes, but failed to check this on reconfiguration.  If a shared memory
   zone was used for another purpose in the new configuration, nginx attempted
   to reuse it and crashed.