pppd: Refactor eap MSCHAPv2 using chap_find_digest

Remove code duplication from eap.c related to eap MSCHAPv2 (server)
implementation.

The chap_find_digest function - introduced in [1] - exposes the
registered CHAP digests type definitions, providing functions to
generate a challenge and verify a response, among others.

1. https://github.com/paulusmack/ppp/commit/14c1a77816e837852138e87a4adccfd66030b2c3

Signed-off-by: Tijs Van Buggenhout <tvbuggen@gmail.com>