Add mutli-pkg support for pkg.latest (yumpkg5)

Also made some docstring tweaks to a few other files.
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().
Merge pull request #3166 from archtaku/issue2906

Add mutli-pkg support for pkg.latest (yumpkg5)