When using a custom http port like 8080, this was resulting in a
constructed hostname of `host.tailnet.ts.net:8080.tailnet.ts.net` when
looking up the serve handler. Instead, strip off the port before adding
the MagicDNS suffix.
Also use the actual hostname in `serve status` rather than the literal
string "host".
Fixes#8635
Signed-off-by: Will Norris <will@tailscale.com>
f8b0caa8c2