client/systray: add installer for a freedesktop autostart file (#18767)

Adds freedesktop as an option for installing autostart desktop files for
starting the systray application.

Fixes #18766

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

client/freedesktop/freedesktop.go

@@ -0,0 +1,43 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package freedesktop provides helpers for freedesktop systems.
+package freedesktop
+
+import "strings"
+
+const needsEscape = " \t\n\"'\\><~|&;$*?#()`"
+
+var escaper = strings.NewReplacer(`"`, `\"`, "`", "\\`", `$`, `\$`, `\`, `\\`)
+
+// Quote quotes according to the Desktop Entry Specification, as below:
+//
+// Arguments may be quoted in whole. If an argument contains a reserved
+// character the argument must be quoted. The rules for quoting of arguments is
+// also applicable to the executable name or path of the executable program as
+// provided.
+//
+// Quoting must be done by enclosing the argument between double quotes and
+// escaping the double quote character, backtick character ("`"), dollar sign
+// ("$") and backslash character ("\") by preceding it with an additional
+// backslash character. Implementations must undo quoting before expanding field
+// codes and before passing the argument to the executable program. Reserved
+// characters are space (" "), tab, newline, double quote, single quote ("'"),
+// backslash character ("\"), greater-than sign (">"), less-than sign ("<"),
+// tilde ("~"), vertical bar ("|"), ampersand ("&"), semicolon (";"), dollar
+// sign ("$"), asterisk ("*"), question mark ("?"), hash mark ("#"), parenthesis
+// ("(") and (")") and backtick character ("`").
+func Quote(s string) string {
+	if s == "" {
+		return `""`
+	}
+	if !strings.ContainsAny(s, needsEscape) {
+		return s
+	}
+
+	var b strings.Builder
+	b.WriteString(`"`)
+	escaper.WriteString(&b, s)
+	b.WriteString(`"`)
+	return b.String()
+}