0.9.13.47: Thread safety miscellania

0.9.13.47: Thread safety miscellania
  * Use localtime_r and gmtime_r instead of thread-unsafe variants
    in time.c.
  * Note gethostby* reentrancy issue in sb-bsd-sockets in BUGS.
  * Trivial PCL cleanups:
    ** *ALL-PV-TABLE-LIST* was unused -- things only got pushed on it,
       but never read. Commented out with a comment, removed code that
       did the pushing.
    ** *INITIAL-PV-TABLE* was unused, deleted.
    ** *EMPTY-PV* was unused, deleted.
    ** *EMPTY-CACHE* was unused, deleted.
    ** *MFP1* & friends implemented a two-element cache outside
       a hash-table. Deleted and replaced with simple hash-table
       access. Boinkmarks will show the truth.
    ** Moved redundant toplevel SETQs to the original DEFPARAMTER
       of various *FOO-COST*s.
    How is this thread-safety related? It happened while flagging
    some PCL globals as safe and others as worthy of attention
    in doc/internals-notes/threading-specials. Also sorted the
    names, so *THE-FOO*s stand out in one place -- seeming to
    me as least-likely causes of trouble.
  * Whitespace.