bring libsolv in line with currrent rpm's multiversion handling

A multiversion install still obeys the obsoletes of a package.
The solver already had a flag to support this, "keepexplicitobsoletes",
but as this is a feature of the package manager it belongs in the
pool, so that the transaction code can do the right thing.
We now have a "noobsoletesmultiversion" flag in the pool that also
defines the default for "keepexplicitobsoletes".