Having two locks in the proc was causing deadlocks.

It is no longer special.

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.