`tailscale netcheck` is the only command that doesn’t support the
`--json` flag, but rather requires `--format=json`. This patch adds a
flag.Value named jsonoutput.Format that handles a boolean `--json`
flag, a versioned `--json=2` flag, and an optional
`--format=json-line` flag.
Updates #17613
Updates #17366
Updates tailscale/corp#33033
Signed-off-by: Simon Law <sfllaw@tailscale.com>