pkg/certutil: Decode could be used more widely

[adamdecaf]

The contents of -file often get read and passed to certutil.Decode, but that function doesn't handle all possible file-based formats. For example java trust stores. Should we expand Decode to handle every format we can?

It would help to simplify some commands for users (i.e. #199), but could add a mess of complexity to implement all that. (Not ever file reader has a usable zero value, aka some, like NSS, require setup.)