Add HKDF key derivation to mungekey

Update mungekey to use the HMAC-based Key Derivation Function (HKDF)
when creating a key.

HKDF is based on the extract-then-expand approach.  During extraction,
entropy_read() (using the kernel's CSPRNG) is used for the input keying
material, while entropy_read_uint() (independent of the kernel's
CSPRNG) is used for the optional salt value.  During expansion, the
use, algorithm, and key length are combined for the optional info
value in order to bind the derived key material to context-specific
information.

Reference:
- https://tools.ietf.org/html/rfc5869
- https://eprint.iacr.org/2010/264
- http://webee.technion.ac.il/~hugo/kdf/

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>