logging RSA key IDs

[Phil Karn]

Hi. To compartmentalize things a bit (e.g., to help limit the damage
should one of my machines be hacked and my private RSA keys stolen) I
use different RSA key pairs on my different client machines.

So it occurs to me that it would be nice if ssh could log which key
was used when logging in to a particular account that has more than
one entry in .ssh/authorized_keys.  Right now it simply says "Accepted
rsa for karn from <blah blah>" without saying which key was used.

You obviously don't want to log the whole public key, just the comment
field from the appropriate line in .ssh/authorized_keys would do.

Phil