Give `-Wformat=0` on illumos/Solaris so that the build does not
complain about `execlp` not having a sentinel (it's nil).  Also
fix an integer size issue when casting to a void pointer.

libthread had code in `_pthreadinit` to test for LinuxThreads,
and error if it was in use.  This hasn't been relevant since
Linux 2.6, and I can't imagine there are many people trying to
build (recent) plan9port on systems that old.

Unfortunately, this code violated aliasing rules, and compilers
were complaining about it.  But, since it likely hasn't had much
relevance in 20ish years, we can probably just remove it.

Use `extern` where appropriate so that we can remove `-fcommon`
on macOS and others.  On macOS, `-fcommon` generates a number of
linker warnings.

I've tested building on macOS, Linux, FreeBSD, NetBSD, OpenBSD
and illumos.  I am not in a position to test on AIX or other
more esoteric platforms, however.

This fixes the 'run stats from rc; exit rc; stats dies' problem.
It's unclear whether this is the right fix or whether rc should
be starting all its interactive commands in their own process
groups. But at least it does fix stats dying.

Having two locks in the proc was causing deadlocks.

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.