Revision 1612405

Follow-up to r1611379: Correctly handle all intermittent errors.

Since the same code sequence has been used for FSFS since 2007,
factor it out into some private API function.

* subversion/include/private/svn_io_private.h
  (svn_io__file_lock_autocreate): Declare new private API.

* subversion/libsvn_subr/io.c
  (svn_io__file_lock_autocreate): Move the file handling code from
                                  the lock() function here and handle
                                  all ordinary and racy errors. 

* subversion/libsvn_subr/named_atomic.c
  (lock): Call the new function and ensure correct thread mutex use.

Found by: brane