Simplify and adjust logging setup

After seeing some of the gratuitous difficulty in debugging for some
people due to lack of output, attempt to streamline and simplify the
logging setup procedure.

There may be a loss here to how well WAL-E composes with other
applications were it used as a library, but there are no such users
known to date nor have they complained, so adjust the trade-off in
favor of a higher chance of getting diagnostics for WAL-E users and
hackers.

@alex, you may be interested in trying this given your confusing diagnostic output.

As it touches every platform and user, a few @wal-e/committers may want to have a skim and a reaction.