telnet.c: rewrite command handling

The previous implementation was unable to handle any command sequences
split across the (maximum) 1024 bytes buffers, leading to crashes,
out-of-bounds access and data corruption.

The previous implementation went into great detail handling plenty of
command strings but only provided stubs debug-printing those commands.

To keep the code concise, the new version prints a hexdump of all
command sequences it extracts from the bytestream (regardless of whether
they're handled or not).

Signed-off-by: Jonas Rebmann <jre@pengutronix.de>

Author: jonrebm