Allow multiple patches

[spoorcc]

In patch management application it is common to have multiple patches. This way each patch represents a single aotmic required change. For instance one for a cve fix, one for feature enhancement etc.

Currently the dfetch manifest only allows a single patch file.

Extend the patch attribute in manifest to also allow a list of strings and apply the patches in order of the manifest.

[spoorcc]

Also support the special case that instead of a patch file a file called series is added, then the directory where it lives is regarded as patch base folder and the series describes the order of patches. This will provide compatibility with quilt for advanced users

[spoorcc]

when prototyping this, I noticed pykwalify cannot handle str | seq(str) for a single entry. Alo it is not rellay maintained. I've tried yamale in #903, but this appears to be not really pain free. Another option is to use strictyaml, the overall design concept seem to align.