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.

If mk gets into a bad state, it's not obvious that you can remove the binary to force the rebuild. Also, not rebuilding means that bugs in mkmk.sh are not noticed. Just rebuild from scratch every time. It doesn't take too long compared to the rest of INSTALL.

Also, if CC9FLAGS includes -fsanitize=address (ASAN), predefine PLAN9PORT_ASAN for use by programs that need to know (mainly libthread). The 9c script used to have a variable called ngflags, which was ccflags except -g (ng stood for "no g"), but nothing needs it split out anymore, so simplify to just ccflags.

Found by ASAN.

getdirentries(2) has been deprecated on macOS since 10.5 (ten releases ago). Using it requires disabling 64-bit inodes, but that in turn makes binaries incompatible with some dynamic libraries, most notably ASAN. At some point getdirentries(2) will actually be removed. For both these reasons, switch to opendir/readdir. A little clunky since we have to keep the DIR* hidden away to preserve the int fd interfaces, but it lets us remove a bunch of OS-specific code too.

More preparation for opendir.

More preparation for opendir.

Preparation for using opendir.

atomics were added in GCC 4.9: https://gcc.gnu.org/gcc-4.9/changes.html

C11 is apparently too new for these systems. Fixes #55.