os: add Process.Kill and Process.Signal

R=alex.brainman, r, rsc, krasin, iant, rsc, r
CC=golang-dev
https://golang.org/cl/4437091
diff --git a/src/pkg/os/exec_unix.go b/src/pkg/os/exec_unix.go
index 8990d6a..cf5ea9b 100644
--- a/src/pkg/os/exec_unix.go
+++ b/src/pkg/os/exec_unix.go
@@ -45,6 +45,14 @@
 	return w, nil
 }
 
+// Signal sends a signal to the Process.
+func (p *Process) Signal(sig Signal) Error {
+	if e := syscall.Kill(p.Pid, int(sig.(UnixSignal))); e != 0 {
+		return Errno(e)
+	}
+	return nil
+}
+
 // Release releases any resources associated with the Process.
 func (p *Process) Release() Error {
 	// NOOP for unix.