upstream commit

Fix public key authentication when multiple
authentication is in use. Instead of deleting and re-preparing the entire
keys list, just reset the 'used' flags; the keys list is already in a good
order (with already- tried keys at the back)

Analysis and patch from Vincent Brillault on bz#2642; ok dtucker@

Upstream-ID: 7123f12dc2f3bcaae715853035a97923d7300176
