allowed adding duplicated local tunnel without warning

[rfree]

I didn't notice MTN is now in router by default (or added it before)
and I re-added MTN.

On same port!

So I have

Name: mtn.i2p2.i2p
Port: 8998
Type: Standard client
Interface: 127.0.0.1

and

Name: mtn
Port: 8998
Type: Standard client
Interface: 127.0.0.1

The UI should block, requite confirm or at least warn when creating another tunel to same interface and port.

(btw, it should know that 127.0.0.1 is [likely] equal to localhost and worn about that as well)

[zzz]

An error isn't appropriate because not all tunnels may be started, dups are ok in that case.

The easiest is to display a warning on the i2ptunnel page, same as we do if the port is not set at all. This is in the tunnel listing, not in the upper message box.

This is only for clients, so no need to check hostnames. Let's ignore the rare case of same port with different listen-on addresses. Servers can have dups, since multiple tunnels may point to the same service.

Warning added in 3d446dbd49feb32a5b3b8ab2c3fbd5d87767b8fb to be 0.9.12-16.