timerfd: Move implementation from linux compat to cpukit/score/src

Move the timerfd impelemntation from linux compat code to cpukit/score/src. 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

cpukit/score/src/kern_tc.c

@@ -86,6 +86,9 @@
 #include <sys/systm.h>
 #endif /* __rtems__ */
 #include <sys/timeffc.h>
+#ifndef __rtems__
+#include <sys/timerfd.h>
+#endif /* __rtems__ */
 #include <sys/timepps.h>
 #include <sys/timetc.h>
 #include <sys/timex.h>
@@ -1585,6 +1588,7 @@ _Timecounter_Set_clock(const struct bintime *_bt,
 
 	/* Avoid rtc_generation == 0, since td_rtcgen == 0 is special. */
 	atomic_add_rel_int(&rtc_generation, 2);
+	timerfd_jumped();
 	sleepq_chains_remove_matching(sleeping_on_old_rtc);
 	if (timestepwarnings) {
 		nanotime(&taft);