Enchance OpenBSD pkg.conf handling

This PR moves the parsing of pkg.conf into a separate method as I intend to use this code for :upgradeable later too. While here I've modified the parser a bit to also handle "installpath += http://some/url/" which was previously ignored. With this it's possible to have a pkg.conf like:

    installpath = foo
    installpath += bar

Which will be turned into a PKG_PATH: foo:bar. The original tests still pass and I've added three new tests to test the new behaviour.