Accept and store the "Internet host number" portion of the telnet

TERMINAL LOCATION NUMBER (RFC 946) in sbbs_t::telnet_location.

Some (older?) versions of fTelnet request to send this option to the BBS to
share the real IP address of the client. Accept this option (when using
"format 0") and store the first 32-bits (an IPv4 address) in the telnet
location storage / caller-ID information. The same place the telnet
SEND LOCATION value is stored if received.

Now, we're not requesting this option (e.g. with a Telnet "DO" command), but
fTelnet doesn't need that: It offers to send it, so we accept.