1.0.15.20: refactor "system locks" framework, one TIMER buglet

1.0.15.20: refactor "system locks" framework, one TIMER buglet

 * Choose the degree of GC/interrupt suppression at compile-time.

 * Default is not to allow interrupts at all, callers which need
   WITH-INTERRUPTS to work can now specify :ALLOW-WITH-INTERRUPTS.

 * Should fix reported Stumpwm crashes due to attempts to recursively
   obtain *SCHEDULER-LOCK*. (Caused by SIGALRM interrupting GET-MUTEX
   inside the call to %TIMER-CANCEL-FUNCTION, which led to recursive
   entry to WITH-SCHEDULER-LOCK.)

 * Don't reschedule timers for dead threads.

 * Three new test-cases for timers, which (1) represent my failing
   attempts to trigger the Stumpwm bug described above (2) led to
   noticing the rescheduling promblem (3) fail horribly on OS X Tiger
   -- not sure if this is our or Darwin's problem...