User-defined default bindings for new threads

This patch adds a previously discussed (long time ago)
*default-special-bindings* feature to SBCL,
whereby users can customize which special variables get automatically
bound in a new thread.

This allows some modularity in that a library L can declare such variables
such that application A can use library L inside threads that
infrastructure I spawns
without infrastructure I having to know anything about L,
or A having to intercept the way I spawns threads.

The patch is only lightly tested, but is rather straightforward.