Received-telnet command improvements:

- If a telnet command was received in multiple packets, the memcpy optimization
  in telnet_interpret() would skip/drop all bytes in the subsequent pkt payload
  before an IAC char. Don't optimize when in the middle of a telnet command.
- If a received telnet command exceeds the telnet_cmd buffer, reset the
  received telnet_cmdlen and log a warning-level message
- If a telnet sub-negotiation END command is received as the beginning of a
  new telnet command, log a warning-level message and reset the telnet_cmdlen.

This fixes the occasional problem observed when using fTelnet and its sending
the "SEND-LOCATION" sub-neg command split between 2 TCP packets. Only
part of the first packet would be used as the location data and the rest
processed as input from the users (e.g. as the Login: prompt). Thanks, Ree!