Updates to yumpkg{,5}.py for multi-pkg support

Updated pkg.available_version and pkg.version to accept an arbitrary
number of arguments. pkg.available_version returns an empty string if
there is no upgrade available for that package, allowing for future
deprecation of pkg.upgrade_available.

Also reverted a change made in 585f7cc that undid a change I had
previously made in bc77261 to speed up list_pkgs().