unix: add SignalNum to convert signal name to a number

Fixes golang/go#28027

Change-Id: Idf8b554e0fd102fd8b2f2f2ea0fa47bf139303da
Reviewed-on: https://go-review.googlesource.com/c/164778
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_unix_test.go

@@ -98,6 +98,27 @@ func TestErrnoSignalName(t *testing.T) {
 	}
 }
 
+func TestSignalNum(t *testing.T) {
+	testSignals := []struct {
+		name string
+		want syscall.Signal
+	}{
+		{"SIGHUP", syscall.SIGHUP},
+		{"SIGPIPE", syscall.SIGPIPE},
+		{"SIGSEGV", syscall.SIGSEGV},
+		{"NONEXISTS", 0},
+	}
+	for _, ts := range testSignals {
+		t.Run(fmt.Sprintf("%s/%d", ts.name, ts.want), func(t *testing.T) {
+			got := unix.SignalNum(ts.name)
+			if got != ts.want {
+				t.Errorf("SignalNum(%s) returned %d, want %d", ts.name, got, ts.want)
+			}
+		})
+
+	}
+}
+
 func TestFcntlInt(t *testing.T) {
 	t.Parallel()
 	file, err := ioutil.TempFile("", "TestFnctlInt")