#46 Add VSA RADIUS attribute mpd-peer-ident

[Cazamir Emil]

I find useful to have the peer_ident value logged by a RADIUS server. Currently (mpd-5.5), only the external_auth mechanism allows access to this value.
Below is a patch which could enable use of a supplemental RADIUS attribute, I've called it 'mpd-peer-ident', with OIDs added to mpd VSA dictionary.
Using this attribute, the server administrator will be able to record the LCP ident string to the RADIUS server.
I haven't tested properly the code, it's just a suggestion