Log keyhandle on auth attempt

[TomMD]

The key handle for a given yubikey should be logged when logging in, such as if there are two keys in the auth file tommd:keyhandle1:...:keyhandle1:... then the auth token used should be apparent from the logs.

[TomMD]

Thoughts?

[LDVG]

Hi, apologies for the delays.

If this is something that would be useful for you and other users, we'd not be opposed to it. I wonder if it would make sense to retrofit into our existing logging/debug facilities (see debug.c and related module options)? That would also solve some of the OpenPAM woes your PR is hitting.