To define a new command, just add a new rule to the gramar:
  CF_CLI(COMMAND NAME, arguments, help-args, help-text) {
	what-should-the-command-do
	} ;
where <arguments> are appended to the RHS of the rule, <help-args> is the
argument list as shown in the help and <help-text> is description of the
command for the help.

<what-should-the-command-do> is a C code snippet to be executed. It should
not take too much time to execute. If you want to print out a lot of
information, you can schedule a routine to be called after the current
buffer is flushed by making cli->cont point to the routine (see the
TEST LONG command definition for an example); if the connection is closed
in the meantime, cli->cleanup gets called.

You can access `struct cli' belonging to the connection you're currently
servicing as this_cli, but only during parse time, not from routines scheduled
for deferred execution.

Functions to call inside command handlers:
  cli_printf(cli, code, printf-args) -- print text to CLI connection,
	<code> is message code as assigned in doc/reply_codes or a negative
	one if it's a continuation line.
  cli_msg(code, printf-args) -- the same for this_cli.

Use 'sock -x bird.ctl' for connecting to the CLI until a client is written.