Fixes a899e14b which changed clamp from 0 to 1 ms which effectivelly
introduced 1ms sleep always, even if requested until_time_ns is in the
past and should request 0 timeout.
While at it also fix mp_poll wrapper to respect negative timeout which
should mean infinite wait.
Also keep the 37d6604 behaviour for very short timeouts, but round only
the ones > 100us, anything else is 0.
Fixes: a899e14b
On linux, several platforms poll for events over a fd. This has ms
accuracy, but mpv's timer is in ns now so lots of precision is lost. We
can use an mp_poll wrapper to use ppoll instead which takes a timespec
directly with nanosecond precision. On systems without ppoll this falls
back to old poll behavior. On wayland, we don't actually use this
because ppoll completely messes up the event loop for some unknown
reason.
Originally, this was added as purely a shim for macOS. However since we
want to do high resolution polling which is not neccesarily available on
all platforms, making this a generic wrapper for poll functions is
useful so rename it.
