cmd/tailscale/cli, util/qrcodes: format QR codes on Linux consoles (#18182)

Raw Linux consoles support UTF-8, but we cannot assume that all UTF-8
characters are available. The default Fixed and Terminus fonts don’t
contain half-block characters (`▀` and `▄`), but do contain the
full-block character (`█`).

Sometimes, Linux doesn’t have a framebuffer, so it falls back to VGA.
When this happens, the full-block character could be anywhere in
extended ASCII block, because we don’t know which code page is active.

This PR introduces `--qr-format=auto` which tries to heuristically
detect when Tailscale is printing to a raw Linux console, whether
UTF-8 is enabled, and which block characters have been mapped in the
console font.

If Unicode characters are unavailable, the new `--qr-format=ascii`
formatter uses `#` characters instead of full-block characters.

Fixes #12935

Signed-off-by: Simon Law <sfllaw@tailscale.com>

util/qrcodes/qrcodes_notlinux.go

@@ -0,0 +1,14 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package qrcodes
+
+import "io"
+
+func detectFormat(w io.Writer, inverse bool) (Format, error) {
+	// Assume all terminals can support the full set of UTF-8 block
+	// characters: (█, ▀, ▄). See tailscale/tailscale#12935.
+	return FormatSmall, nil
+}