At least in neovim, `│vi:` is not recognized as a modeline because it
has no preceding whitespace. After fixing this, opening a file yields
an error because `net` is not an option. (`noet`, however, is.)
- enable WITH_THREAD and _POSIX_THREADS
- add headers everywhere
- breaks only two tests (faulthandler and signal)
- disabled terminal completion because it causes segfaults for some
reason (probably could not get the current thread)
We remove (i.e. hide behind a debug ifdef) the recursion checking methods,
and the memory hooks and memory allocator methods. ASAN mode has no
PYMALLOC, so we need a macro. Fix build break with des.c stack allocation.
