Add EncFS package to GNU Guix

[Tobias Geerinckx-Rice]

Hullo Guix,

Good news! My hard drive made a loud grinding noise this weekend. Now I
find myself installing GuixSD on bare metal, probably for good this time.

Also, testing my back-ups, which I am told is a good thing.

Some of these were still[1] encrypted using EncFS, which was missing from
Guix. Here are some patches to fix that.

Kind regards,

T G-R

[1]: It was unmaintained for a while and ha{d,s} some properties that
     make it a poor choice in some cases. I consider it fine for home
     use. Ask your doctor if EncFS is right for you.