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/zerrors_linux.go

@@ -1268,6 +1268,9 @@ const (
 	IP_XFRM_POLICY                              = 0x11
 	ISOFS_SUPER_MAGIC                           = 0x9660
 	ISTRIP                                      = 0x20
+	ITIMER_PROF                                 = 0x2
+	ITIMER_REAL                                 = 0x0
+	ITIMER_VIRTUAL                              = 0x1
 	IUTF8                                       = 0x4000
 	IXANY                                       = 0x800
 	JFFS2_SUPER_MAGIC                           = 0x72b6