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>
dbaf0b0472