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.

Suggested by G. Brandon Robinson.

Drawing as white on black to produce a mask only works if the white on black is the inversion of black on white. Emoji that force use of specific colors don't respect that. Draw black on white and invert to mask separately.

The overloading of IR emits magic \X'...' sequences that turn into HTML manual links. But not all such IR invocations should be manual links; those had to be written to avoid the IR macro before. Worse, the \X'...' ending the IR causes troff to emit only a single space after a period. Defining a new IM macro for manual references fixes both problems. Fixes #441.

This fixes https://github.com/9fans/plan9port/issues/436 This doesn't necessarily address the underlying issue: calling p9create with mode = OREAD should probably be allowed, but currently doesn't work on OpenBSD.

Fixes #432.