CLI fix for long-lived sessions during high loads

CLI fix for long-lived sessions during high loads

When there is a continuos stream of CLI commands, cli_get_command()
always returns 1 (there is a new command). Anyway, the socket receive
buffer was reset only when there was no command at all, leading to a
strange behavior: after a while, the CLI receive buffer came to its end,
then read() was called with zero size buffer, it returned 0 which was
interpreted as EOF.

Fixing this by:
* resetting the buffer any time CLI RX gets to EOL
* explicitly refusing to pipeline

In future, we may implement CLI pipelining, yet to make it conveniently,
a push-parser may come handy instead of the current implementation.