ikev2: Add support to switch peer configs based on EAP-Identities

ikev2: Add support to switch peer configs based on EAP-Identities

This changes how EAP identities are used from the config. Instead of
setting a statically configured identity != %any, an EAP-Identity
exchange is now always initiated (and required).  If the received identity
doesn't match, the peer config is switched to one with a matching
identity (wildcards are supported for that match).  This allows switching
to a config with a different EAP method or child settings based on the
EAP identity.

There is currently no "best" match.  The configs are evaluated based on
the order returned from the initial peer config lookup.

References strongswan/strongswan#2702