cmd/derper: add flag to run derper in bootstrap-dns-only mode

Change-Id: Iba128e94464afa605bc9df1f06a91d296380eed0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f