Now that we assume pthreads, the only assembly left is in libmp and libsec. We only ever added assembly for 386. The portable C code is fine for plan9port.

Fixes #376.

Fixes #362.

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.

I added a direct call from thread.c to pthread.c's _threadpthreadstart in May, and no one has complained about NetBSD being broken. So probably no one is using this on NetBSD at all. Make pthread the only option.

Fixes #469.

This was causing sporadic but frequent crashes at startup in 9pserve on the new M1 Macs, correctly diagnosing a use-after-free.

This should make the AIX build finally work. Fixes #400.