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.

Also reduce duplication: makecontext is per-arch not per-os-arch. May fix #353.

Linux.c was for Linux 2.4 and is no longer used directly, only indirectly because NetBSD.c was a 1-line file #including Linux.c. So mv Linux.c NetBSD.c. Also rm Linux-*-asm.s which was for Linux 2.4 as well.

Pretty sure FreeBSD 4 is gone now. :-)

R=rsc http://codereview.appspot.com/6401049

R=rsc http://codereview.appspot.com/4838047