EAP peer: Add a concept of a separate machine credential

EAP peer: Add a concept of a separate machine credential

This is an initial step in adding support for configuring separate user
and machine credentials. The new wpa_supplicant network profile
parameters machine_identity and machine_password are similar to the
existing identity and password, but explicitly assigned for the purpose
of machine authentication.

This commit alone does not change actual EAP peer method behavior as
separate commits are needed to determine when there is an explicit
request for machine authentication. Furthermore, this is only addressing
the username/password credential type, i.e., additional changes
following this design approach will be needed for certificate
credentials.

Signed-off-by: Jouni Malinen <j@w1.fi>