In bootstrap and standalone-backend modes, do not sort LOG elevel out

In bootstrap and standalone-backend modes, do not sort LOG elevel out
of order; the 'server log' output is actually client output in these
scenarios and we ought to treat elevels the same way as in the client
case.  This allows initdb to not send backend stderr to /dev/null anymore,
which makes it much more likely that people will notice problems during
initdb.