1.0.4.3: interrupt and GC issues

1.0.4.3: interrupt and GC issues
 * Add WITHOUT-INTERRUPTS to WITHOUT-GCING.
 * Warn if WITH-INTERRUPTS nested in WITHOUT-GCING.
 * Make sure that SIG_STOP_FOR_GC and SIG_RESUME_FROM_GC are enabled on
   threaded builds before calling into SUB-GC from the runtime.
 * Better WITHOUT-GCING, WITHOUT-INTERRUPTS, and WITH-INTERRUPTS
   documentation.
 * Internals documentation about POSIX signal safety rules.