make sup-sync's --start-at actually useful

make sup-sync's --start-at actually useful

Using --start-at with sup-sync will now seek forward to the next valid message
if given an offset that's not on a message boundary. This makes it possible to
use that option without going through the laborious process of finding the
exact message boundary.

Also die unless --start-at is used with only one source, since using it across
multiple sources is almost definitely an error. (And if that's really what you
want, you can just call sup-sync multiple times.)