telnet flood protection doesnt cover multiple hosts

[michaelortmann]

dcc.c:detect_telnet_flood() only matches last seen host. A telnet flood from user A is not detected, if in the meantime, user B telnets the bot. In other words, someone with control over at least 2 hosts can circumvent the telnet flood protection.

Fixing this requires switching from last seen host logic to a bigger data structure. This issue would make for a good first time PR for a new eggdrop coder.