Redoing conditional thread support

[Marius Vollmer]

Hi,

I'm about to reorganize our conditional threading stuff so that there
is always a thread API (scm_current_thread, scm_call_with_new_thread,
mutices, condition variables, etc), but when you configure
"--without-threads" you get a null-thread implementation that doesn't
allow the creation of new threads.  Thus there will always be only a
single thread.

This should make it easier to write code that is thread aware but
doesn't require threads, like the thread-aware sigaction function.

Objections?