Revert the side effect of 7c40996c.
Do not convert password hash to its binary representation when a user
entry is loaded. Do it lazily on the first authenticatation attempt.

As a collateral - force all authentication plugins to follow the
protocol and read_packet at least once before accessing info->username
(username is not available before first client handshake packet is read).

Fix PAM and GSSAPI plugins to behave.