util: password: Add a cancellable ReadPassword style functions

It was non-trivial to do this, so I put it into a library. Strangely I
couldn't directly wrap the ReadPassword function from the
golang.org/x/term package, as it wouldn't unblock for some reason.

util/password/password_test.go

@@ -0,0 +1,68 @@
+// Mgmt
+// Copyright (C) 2013-2023+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+//go:build !root
+
+package password
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/signal"
+	"sync"
+	"syscall"
+	"time"
+)
+
+func ExampleReadPasswordCtx() {
+	// Put this in a main function and it will not have the ioctl error!
+	fmt.Println("hello")
+	defer fmt.Println("exiting...")
+
+	wg := &sync.WaitGroup{}
+	defer wg.Wait()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// If we exit without getting a chance to reset the terminal, it might
+	// be borked!
+	ch := make(chan os.Signal, 1+1) // must have buffer for max number of signals
+	signal.Notify(ch, syscall.SIGTERM, os.Interrupt)
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		select {
+		case <-ch:
+			cancel()
+		case <-ctx.Done():
+		}
+	}()
+
+	password, err := ReadPasswordCtx(ctx)
+	if err != nil {
+		fmt.Printf("error: %+v\n", err)
+		return
+	}
+
+	fmt.Printf("password is: %s\n", string(password))
+
+	// Output: hello
+	// error: inappropriate ioctl for device
+	// exiting...
+}