Use unix.SignalNum in ParseSignal on unix platform

This removes the signalMap on unix platform, since
the signalMap on different architectures is not same,
especially it's wrong on mipsx.

golang.org/x/sys/unix now has a SignalNum func to convert
signal name to a number, thus there's no need to keep
this redundant map.

Windows platform still needs to have a signalMap, since
golang.org/x/sys/windows doesn't have corresponding
functions.

Address: #3061

Signed-off-by: Shengjing Zhu <zhsj@debian.org>

signals_test.go

@@ -0,0 +1,47 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package containerd
+
+import (
+	"fmt"
+	"syscall"
+	"testing"
+)
+
+func TestParseSignal(t *testing.T) {
+	testSignals := []struct {
+		raw  string
+		want syscall.Signal
+		err  bool
+	}{
+		{"1", syscall.Signal(1), false},
+		{"SIGKILL", syscall.SIGKILL, false},
+		{"NONEXIST", 0, true},
+		{"65536", 0, true},
+	}
+	for _, ts := range testSignals {
+		t.Run(fmt.Sprintf("%s/%d/%t", ts.raw, ts.want, ts.err), func(t *testing.T) {
+			got, err := ParseSignal(ts.raw)
+			if ts.err && err == nil {
+				t.Errorf("ParseSignal(%s) should return error", ts.raw)
+			}
+			if !ts.err && got != ts.want {
+				t.Errorf("ParseSignal(%s) return %d, want %d", ts.raw, got, ts.want)
+			}
+		})
+	}
+}