unix: implement Waitid on Linux

waitid(2) can be used to wait on pidfds. Update TestPidfd to test this new
functionality as well.

Change-Id: I5f26140b9afd5a6ec3f04217fb1c91542936a2c3
Reviewed-on: https://go-review.googlesource.com/c/sys/+/395936
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_linux.go

@@ -366,6 +366,8 @@ func Wait4(pid int, wstatus *WaitStatus, options int, rusage *Rusage) (wpid int,
 	return
 }
 
+//sys	Waitid(idType int, id int, info *Siginfo, options int, rusage *Rusage) (err error)
+
 func Mkfifo(path string, mode uint32) error {
 	return Mknod(path, mode|S_IFIFO, 0)
 }
@@ -2446,5 +2448,4 @@ func Setitimer(which ItimerWhich, it Itimerval) (Itimerval, error) {
 // Vfork
 // Vhangup
 // Vserver
-// Waitid
 // _Sysctl