BUG/MINOR: master/cli: only parse the '@@' prefix on complete lines

BUG/MINOR: master/cli: only parse the '@@' prefix on complete lines

The new adhoc parser for the '@@' prefix forgot to require the presence
of the LF character marking the end of the line. This is the reason why
entering incomplete commands would display garbage, because the line was
expected to have its LF character replaced with a zero.

The problem is well illustrated by using socat in raw mode:

   socat /tmp/master.sock STDIO,raw,echo=0

then entering "@@1 show info" one character at a time would error just
after the second "@". The command must take care to report an incomplete
line and wait for more data in such a case.