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_unix.go

@@ -0,0 +1,47 @@
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+/*
+   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"
+	"strconv"
+	"strings"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// ParseSignal parses a given string into a syscall.Signal
+// the rawSignal can be a string with "SIG" prefix,
+// or a signal number in string format.
+func ParseSignal(rawSignal string) (syscall.Signal, error) {
+	s, err := strconv.Atoi(rawSignal)
+	if err == nil {
+		signal := syscall.Signal(s)
+		if unix.SignalName(signal) != "" {
+			return signal, nil
+		}
+		return -1, fmt.Errorf("unknown signal %q", rawSignal)
+	}
+	signal := unix.SignalNum(strings.ToUpper(rawSignal))
+	if signal == 0 {
+		return -1, fmt.Errorf("unknown signal %q", rawSignal)
+	}
+	return signal, nil
+}