Fix the build for DragonFlyBSD.

glibc will only expose sigjmp_buf if __USE_POSIX is defined.

Now that everything uses pthreads and pthreadperthread,
can delete various conditionals, all the custom context code,
and so on. Also update documents.

Fixes #355.

Programs that want to background themselves now need
to define threadmaybackground returning 1.
This avoids a confusing (to people and debuggers)
extra parent process for all the threaded programs
that will never want to background themselves.

With very large fonts (72pt or so) I see bad cache glyphs,
and this fixes it. Not entirely sure exactly which code
is overflowing, but something is.