bhyveload: Address compiler warnings and bump WARNS

Mostly sprinkle __unused. Also duplicate strings passed to addenv() so
that constant string literals can be passed. No functional change
intended.

Reviewed by: corvink, rew, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38840

(cherry picked from commit ad43dd69cec7b568846b45ff990da6dc86e58467)