Make additional parts of sys/geom/eli more usable in userspace

The upcoming GELI support in the loader reuses parts of this code
Some ifdefs are added, and some code is moved outside of existing ifdefs

The HMAC parts of GELI are broken out into their own file, to separate
them from the kernel crypto/openssl dependant parts that are replaced
in the boot code.

Passed the GELI regression suite (tools/regression/geom/eli)
Files=20 Tests=14996
Result: PASS

Reviewed by: pjd, delphij
MFC after: 1 week
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D4699