net: telnet: Improve command processing

Current TELNET implementation wrongly assumed that if the command is
present, it will always be included at the beginning of the packet/data
buffer. Such assumptions however are not valid for STREAM sockets,
where the actual stream data could be fragmented in any way.

Therefore, the command parsing needed rework, so that we analyze each
byte received for the command escape code, and once received we enter
"command parsing" mode. Once no more commands are identified in a data
streak, the command bytes are removed from the data buffer before the
buffer is provided to the shell subsystem for processing.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>