Fix connecting via Host Key

[fizzxed]

Previously if we passed a ssh host key, e.g. zmosh attach -r foo dev
where the ssh config file looked like

Host foo
    HostName actual.host.address

It would fail to connect due to trying to connect to foo after the ssh bootstrap

[ww7]

Backward compatibility issue with ZMX_CONNECT protocol change

This breaks compatibility when the client and server are running different versions. If an updated client connects to an older server (or vice versa), the parser fails with InvalidPort because it misinterprets
the fields.

Additionally, connectRemote returns result.host which is a slice into a stack-local buffer (buf[512]). Once the function returns, this becomes a dangling pointer — a use-after-free that causes the UDP connection
to target a garbage address and hang.

Suggested fix: Make parseConnectLine detect both formats by checking whether the 4th field parses as a port number. If it does, treat field 3 as host (new format); otherwise fall back to the old format (field 3 = port, field 4 = key). When the host comes from the connect line, alloc.dupe it to avoid the dangling pointer. When using the old format, fall back to the SSH host argument.

[fizzxed]

Down to fix with the suggestion if backwards compatibility is an issue. Considering This isn't yet version 1.0 I didn't put a thought to that.