Liberalize another assertion for widetag range compression test

The previous fix in commit 98c61848748078bd412af0f03ade50dcf85170eb
missed the

  (or system-area-pointer (sb-kernel:simple-unboxed-array (*)))

case which was failing for sb-safepoint builds as well.
Add SB-THREAD:INTERACTIVE-THREADS

Also have SB-THREAD:FOREGROUND-THREAD accept the session as an optional
parameter.
Document INTERACTIVE-THREADS, FOREGROUND-THREAD
SAVE-LISP-AND-DIE signals SAVE-WITH-MULTIPLE-THREADS-ERROR

The report of the new condition class SAVE-WITH-MULTIPLE-THREADS-ERROR
contains the interactive threads of the current session and all other
threads to make analyzing the problem easier.

Also define superclass conditions SAVE-{CONDITION,ERROR}.
Undo the change that added a %FUN-POINTER-WIDETAG reader.

It's just FUN-SUBTYPE, though questionable why SAP-REF-8
is getting re-implemented anew by each backend.
Fix lp# 1321047 - problem with #. due to change 392e2b25