Two changes are needed in monitor_handle_command() so that it
knows how to work in control mode.
First, it has to know whether a given command is part of the
protocol or not. Second, it has to print the correct server
response when a command execution finishes.
A good approach to do this would be:
1. Add a new member to struct mon_cmd_t, which will tell if
the command is already part of the protocol.
2. Change handler_* functions to return exit status, so that
monitor_handle_command() can print "+ OK" or "- ERR"
accordling.