swapon: parse md.eli device for encrypted swapfile from fstab

Summary:
Although fstab(5) suggests usage of md(4) and an .eli suffix in the same
paragraph, the combination did not work.
This patch adds a bit of parsing, creates the md device and then passes
that device to the *_geli function.

Add unit tests.

PR: 186252
Approved by: delphij
MFC after: 2 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D48874

(cherry picked from commit 1ee33e3a578c35ff0e8ef5db87a8005f1f99546a)