Interworking: Add GET_CRED ctrl_iface command

Interworking: Add GET_CRED ctrl_iface command

"GET_CRED <id> <field>" can now be used to fetch credential parameters
over the control interface. This does not allow passwords etc. private
material to be retrieved ("*" is returned for those if the value is set
regardless of the value). FAIL is returned if the requested parameter
has not been set. For cred parameters that can have multiple values,
newline separated list of values is returned.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>