Document more cleanup.
Default xcreate/xopen to O_CLOEXEC. (Pass O_CLOEXEC in the flags to switch it back off.)

This way we're not leaking filehandles to children from things like find -exec.
Move DIRTREE_COMEAGAIN second callback up to when the filehandle is still open, and add dir->again variable to distinguish second call instead of checking for -1 filehandle.
Patch from Isaac Dunham to work around the deficiencies in musl's regex engine (which break building under alpine linux).

Musl's regex engine doesn't support \| which changes the behavior of busybox
sed, breaks toybox grep... I need to come up with a musl patch, in the meantime
here's a workaround.