libthread: drop schedlock Having two locks in the proc was causing deadlocks.

libthread: simplify Now that everything uses pthreads and pthreadperthread, can delete various conditionals, all the custom context code, and so on. Also update documents. Fixes #355.

libthread: make pthreadperthread the default

libthread: fix pthreadperthread bugs

libthread: add threadmaybackground 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.

libthread: fix use after free of first thread in each proc This was causing sporadic but frequent crashes at startup in 9pserve on the new M1 Macs, correctly diagnosing a use-after-free.

libthread: handle spurious _procsleep wakeups, fix $LIBTHREAD handling

libthread: add pthreadperthread mode and use under ASAN 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.

libthread: comment stack border a bit more

libthread: use mmap to allocate OpenBSD stacks Should fix faults on OpenBSD. Fixes #218. Fixes #226.

libthread: fix fault in teardown of proc Fixes #332.

libthread: run first thread in proc on system stack 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.

Trivial changes: whitespace and modes. 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>

libthread: Lion context routines

build: OS X 64-bit build R=rsc http://codereview.appspot.com/4838047