Foreign callbacks

Foreign callbacks

Allow alien callbacks to be invoked in pthreads created outside of
the Lisp runtime:

Add new runtime functions attach_os_thread, detach_os_thread
allowing such threads to acquire a `struct thread' temporarily,
turning them into Lisp threads.

In a main deviation from the Windows branch (which has a similar
feature), this mechanism does not involve user-land thread (fiber)
mechanisms to switch between stacks.  Instead, Lisp code merely runs
on the existing pthread's stack.

Currently a safepoint-only feature, because only safepoint-based
builds already go through a convenient trampoline function for
callbacks, but a backport of this feature to non-safepoint builds
might be straightforward.