VPP-27 Extend telnet protocol buffer guard

VPP-27 Extend telnet protocol buffer guard

With Telnet protocol overhead, the string "xterm-256color", which is the
norm when the terminal is iTerm on a Mac, is longer than the 16 byte
lookahead overflow guard in the Telnet protocol processor.

This extends it to 24 bytes.

This guard is designed to encourage the protocol processor to quit
waiting for bytes indefinitely if those already in the buffer do not
form a complete message. Whilst this is unlikely, extending the guard
length would mean more bytes need to be received before it gives up.

Change-Id: Ibaa3d35b78bfd298fe0e4f4c6e508440f122e916
Signed-off-by: Chris Luke <chrisy@flirble.org>