For people who can't use LetsEncrypt because it's banned.
Per https://github.com/tailscale/tailscale/issues/11776#issuecomment-2520955317
This does two things:
1) if you run derper with --certmode=manual and --hostname=$IP_ADDRESS
we previously permitted, but now we also:
* auto-generate the self-signed cert for you if it doesn't yet exist on disk
* print out the derpmap configuration you need to use that
self-signed cert
2) teaches derp/derphttp's derp dialer to verify the signature of
self-signed TLS certs, if so declared in the existing
DERPNode.CertName field, which previously existed for domain fronting,
separating out the dial hostname from how certs are validates,
so it's not overloaded much; that's what it was meant for.
Fixes#11776
Change-Id: Ie72d12f209416bb7e8325fe0838cd2c66342c5cf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
