Say how a file is protected and what it allows

[tannevaled]

A document that opened has already been decrypted, and said nothing about what it had been protected with on the way.

Document.Protection() now reports it:

type Protection struct {
    Method      string      // "RC4-40", "RC4-128", "AES-128", "AES-256", "none"
    Revision    int         // 2, 3, 4, 5 or 6
    Permissions Permissions // what the file grants
    Owner       bool        // opened with the owner password, so they do not apply
}

Working out which password opened a file meant carrying that back from both key derivations, so a legacy candidate now says whether reaching it meant knowing the owner's password rather than the user's.

Permissions gains Allows and String, so what a file grants can be put in front of a person as words — print, copy — rather than as a bit pattern.

This is what ops needs for a permissions verb, and what pdfops needs to answer "what is this file, and what may I do with it".

100% statement coverage, go vet and gofmt clean.