bin/cookies: Drop libsoup code, fix fd-relative issues, new style

bin/cookies: Drop libsoup code, fix fd-relative issues, new style

Prep for `ostree_repo_new_at()`. These commands were directly accessing
`repo->repodir`, which it turns out was unnecessary since the the APIs they then
used were fd-relative. Except actually there were bugs there, so fix all of the
cookie util code to actually use the passed `dfd` and not just hardcode
`AT_FDCWD`.

Also, libsoup can't handle this (its APIs require fully qualifed paths), and
there's not a really good reason to have two implementations now; historically
it was useful to cross-check them, but I don't think we need that.

While I'm here, port to new style.

Closes: #1010
Approved by: jlebon