CLIHandler: fix command usage string generation

CLIHandler: fix command usage string generation

track our command string, i.e. everything which cannot be an argument
for a specific command, in resolve_cmd, as we go through the commando
definition there anyway and know if a ARGV element is part of the
command itself or its arguments.

Fixes a problem where a invalid command had all the passed parameter
attached in the resulting USAGE output.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>