unix: implement getitimer(2) and setitimer(2) on Linux

Wrap the low-level system calls with a more idiomatic Go API and a set of
constants to indicate which timer should be queried or modified.

man 2 getitimer indicates that these system calls are obsolete as of
POSIX.1-2008, but the code I am working on has not been ported to the modern
timer_gettime(2) and timer_settime(2) APIs as of yet.

Change-Id: I91482e141047846cadf47aa2417b8770955986bf
Reviewed-on: https://go-review.googlesource.com/c/sys/+/384054
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dominik Honnef <dominik@honnef.co>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Matt Layher <mdlayher@gmail.com>

unix/linux/types.go

@@ -459,6 +459,8 @@ type Timex C.struct_timex
 
 type ItimerSpec C.struct_itimerspec
 
+type Itimerval C.struct_itimerval
+
 const (
 	TIME_OK    = C.TIME_OK
 	TIME_INS   = C.TIME_INS