client: "agent" command capability argument and autocompletion
client: "agent" command capability argument and autocompletion

This patch enables argument autocompletion for the agent command with
the list of capabilities an agent can have, adding also "on" (for the
default "") and "off". The command passes the argument (parsing and
verifying it) to the dbus method call.

mapfau/bluez,mapfau/bluez,ComputeCycles/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,pkarasev3/bluez,silent-snowman/bluez,mapfau/bluez,ComputeCycles/bluez,silent-snowman/bluez,silent-snowman/bluez,silent-snowman/bluez,pkarasev3/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez