Fix authentication so that it doesn't record an extra 'Password

Fix authentication so that it doesn't record an extra 'Password
authentication failed' and a 'send() failed: Broken pipe' message
on every connection from psql in password auth mode.  Problem is
that psql doesn't ask user for a password until it sees a password
challenge failure, and libpq just closes the connection unceremoniously
if it's challenged for a password when it hasn't got one to send.
Accordingly, EOF from the client after asking for a password is
normal behavior and should not result in postmaster log entries.