unix: add PidfdSendSignal on Linux

This completes the pidfd_* family of syscalls and adds a test to verify that a
child process can be terminated using the new PidfdSendSignal function.

Change-Id: I71f6a1e26518f513731e20afef02030462e9a2c0
Reviewed-on: https://go-review.googlesource.com/c/sys/+/393794
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>

unix/linux/types.go

@@ -934,6 +934,8 @@ const _C__NSIG = C._NSIG
 
 type SignalfdSiginfo C.struct_signalfd_siginfo
 
+type Siginfo C.siginfo_t
+
 // Terminal handling
 
 type Termios C.termios_t