1.0.29.17: SYMBOL-VALUE-IN-THREAD

1.0.29.17: SYMBOL-VALUE-IN-THREAD

* Build on top of %SYMBOL-VALUE-IN-THREAD, document and export from
  SB-THREAD. Write a bunch of test-cases.

* New condition: SB-THREAD:THREAD-ERROR. Inherit from it in
  JOIN-THREAD-ERROR and INTERRUPT-THREAD-ERROR, and deprecate
  JOIN-THREAD-ERROR-THREAD and INTERRUPT-THREAD-ERROR-THREAD in favor
  of THREAD-ERROR-THREAD.

* General threading related documentation touchups.