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.

ASAN can't deal with the coroutine stacks. In theory we can call into ASAN runtime to let it know about them, but ASAN still has problems with fork or exit happening from a non-system stack. Bypass all possible problems by just having a full OS thread for each libthread thread. The threads are still cooperatively scheduled within a proc (in thos mode, a group of OS threads). Setting the environment variable LIBTHREAD=pthreadperthread will enable the pthreadperthread mode, as will building with CC9FLAGS='-fsanitize=address' in $PLAN9/config. This solution is much more general than ASAN - for example if you are trying to find all the thread stacks in a reproducible crash you can use pthreadperthread mode with any debugger that knows only about OS threads.

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

More dead code.

Pretty sure FreeBSD 4 is gone now. :-)

Should fix faults on OpenBSD. Fixes #218. Fixes #226.

Fixes #332.

For pthread systems that are fussy about which stack is used, this makes sure that threadmain runs on a system stack. If you only use proccreate (never threadcreate), all threads run on system stacks.

Remote whitespace at the ends of lines. Remove blank lines from the ends of files. Change modes on source files so that they are not executable. Signed-off-by: Dan Cross <cross@gajendra.net>

R=rsc http://codereview.appspot.com/6742064

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