strongswan: Add support for EAP-MSCHAPv2 authentication

strongswan: Add support for EAP-MSCHAPv2 authentication

Support for EAP-MSCHAPv2 authentication scheme is added.

Different from the previously supported schemes, this one is
usually asymmetric in the way that server auth method (pubkey) is
different from the client auth method (eap-mschapv2).
The code handles this asymmetry automatically.

A new UCI config section mschapv2_secrets is added where the user
can specify the EAP identities and their passwords that are
accepted by the server. AFAIK, there is no way to select which
EAP IDs should be accepted by which remote, except setting
`eap_id` to something different than `%any`. But `eap_id`
does not support template matching, so either only a single
identity or all can be configured for one remote. This is why
the EAP identities are not subsections of remotes, but are
a standalone section.

Signed-off-by: Martin Pecka <peci1@seznam.cz>
Signed-off-by: Martin Pecka <peckama2@fel.cvut.cz>