timerfd: Move kern_timerfd_* prototypes to <sys/syscallsubr.h>

timerfd: Namespace pollution adjustments

Do not pollute userspace with <sys/proc.h>, instead declare struct thread
when _KERNEL is defined.

Include <sys/time.h> instead of <sys/timespec.h>. This causes intentional
namespace pollution that mimics Linux. g/musl libcs include <time.h> in
their <sys/timerfd.h>, exposing clock gettime, settime functions and
CLOCK_ macro constants. Ports like Chromium expect this namespace
pollution and fail without it.

MFC After: 3 days
Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D41641

timerfd: Move implementation from linux compat to sys/kern

Move the timerfd impelemntation from linux compat code to sys/kern. Use
it to implement the new system calls for timerfd. Add a hook to kern_tc
to allow timerfd to know when the system time has stepped. Add kqueue
support to timerfd. Adjust a few names to be less Linux centric.

RelNotes: YES
Reviewed by: markj (on irc), imp, kib (with reservations), jhb (slack)
Differential Revision: https://reviews.freebsd.org/D38459