tools: Add frob-catmsgs tool for sending raw messages to agent

Specify a directory of files, each which is a message, complete
with the channel prefix. frob-catmsgs will send the correct
MSB length prefix.

See doc/protocol.md for more details

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
agent: Fix several leaks in cockpit-agent

The functions json_builder_add_value(), json_object_set_member() and
json_builder_get_root() have unexpected ownership characteristics.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>