Late time64 changes on the way

[Rich Felker <dalias@libc.org>]

Based on recent findings, I'm working on some late (in the sense of
late in the release cycle) time64 changes I didn't expect to be
making. In particular it was found that, without significant
time64-aware hacks on the application/other-libraries side, the
following would stop working when built for time64:

- socket timestamping (SO_TIMESTAMP[NS])
- sound (ALSA)
- video capture (V4L2)
- ppp (timeouts)
- legacy parallel ports (¯\_(ツ)_/¯)

The first 3, and possibly the first 4, are a pretty big deal -- "Sound
isn't going to work anymore unless you upgrade your kernel" is a big
blocker to adoption and would almost certainly leave us with a
situation where distros/users are trying to stick back with pre-time64
versions or use tricks to keep building new binaries as time32.

I already have a fix for the first queued (fallback to old options
that produce 32-bit timestamps with translation of the cmsg payload in
recvmsg) and an in-progress fix for the rest.

For sound, there are still pending patches to the kernel headers that
will be needed for it to work right:

https://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git/log/?h=y2038-endgame

The needed ones will be backported/merged in musl-cross-make soon.

Once I push these changes, I'd love some assistance testing that they
actually work from users who have appropriate setups (32-bit userspace
with pre-time64 kernel, sound or video capture hardware, software for
using it). Please reply (or mention it on #musl) if you can help.

Rich