revert following; deals badly with agent keys

revert following; deals badly with agent keys

revision 1.285
date: 2018/09/14 04:17:12;  author: djm;  state: Exp;  lines: +47 -26;  commitid: lflGFcNb2X2HebaK;
Use consistent format in debug log for keys readied, offered and
received during public key authentication.

This makes it a little easier to see what is going on, as each message
now contains the key filename, its type and fingerprint, and whether
the key is hosted in an agent or a token.