Implement os.Executable

For now this is a stub for everything but linux, which is a slightly
modified copy of the official implementation.

Should address #1778.

src/os/executable_other.go

@@ -0,0 +1,9 @@
+// +build !linux
+
+package os
+
+import "errors"
+
+func Executable() (string, error) {
+	return "", errors.New("Executable not implemented")
+}

src/os/executable_procfs.go

@@ -0,0 +1,25 @@
+// The following is copied from Go 1.17 official implementation.
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build linux
+
+package os
+
+func Executable() (string, error) {
+	path, err := Readlink("/proc/self/exe")
+
+	// When the executable has been deleted then Readlink returns a
+	// path appended with " (deleted)".
+	return stringsTrimSuffix(path, " (deleted)"), err
+}
+
+// stringsTrimSuffix is the same as strings.TrimSuffix.
+func stringsTrimSuffix(s, suffix string) string {
+	if len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix {
+		return s[:len(s)-len(suffix)]
+	}
+	return s
+}