client/main: add help option for available args

client/main: add help option for available args

This option exists to facilitate external completion of commands that
have specially defined arguments. It effectively adds two new commands
to bluetoothctl with the following output:

$ bluetoothctl agent help
on
off
DisplayOnly
DisplayYesNo
KeyboardDisplay
KeyboardOnly
NoInputNoOutput

$ bluetoothctl advertise help
on
off
peripheral
broadcast

Shell completion scripts can then parse this output to provide
completions for those commands.

Change-Id: Ia431ea11d7a1cb344b512f0180bd0ebac0754567
Signed-off-by: himanshu <h.himanshu@samsung.com>