timerfd_create: accept CLOCK_UPTIME/CLOCK_BOOTTIME

This is a common use case when using timerfd_create to actually use
it with CLOCK_BOOTTIME on linux which is CLOCK_UPTIME for us.

Note that currently on freebsd CLOCK_BOOTTIME is CLOCK_UPTIME, but the
semantic is supposed to be different, this has to be fixed later.

Tested with the fnott notification software

Reviewed by: des, imp
Differential Revision: https://reviews.freebsd.org/D44253