modemmanager: allow specifying list of authentication protocols

modemmanager: allow specifying list of authentication protocols

ModemManager allows specifying which are the authentication protocols
to be used during the user/password context authentication with the
peer.

This protocol update allows users to provide a new 'allowedauth'
option in the interface configuration, which is then used in two
different places:
 * It is sent to ModemManager in the --simple-connect call so that
   modems with a network interface can perform the authentication
   using their own vendor-specific protocol.
 * If the connection is done using PPP, this list of protocols is used
   to configure the pppd call.

If the new 'allowedauth' option is not given, all auth protocols are
implicitly allowed.

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>