sort: use mutexes, not spinlocks (avoid busy loop on blocked output)

sort: use mutexes, not spinlocks (avoid busy loop on blocked output)

Running a command like this on a multi-core system
  sort < big-file | less
would peg all processors at near 100% utilization.
* src/sort.c: (struct merge_node) Change member lock to mutex.
All uses changed.
* tests/Makefile.am (XFAIL_TESTS): Remove definition, now that
this test passes once again.  I.e., the sort-spinlock-abuse test
no longer fails.
* NEWS (Bug reports): Mention this.
Reported by DJ Lucas in http://debbugs.gnu.org/7489.