Improve parameters handling in veriexec

Provide more robust parameter parsing in veriexec. Do a little cleanup as well.

Differential revision: https://reviews.freebsd.org/D33246
Obtained from: Semihalf
Reviewed by: sjg, sebastien.bini_stormshield.eu

(cherry picked from commit b439f64ac1b953936a89167f0201d0d53cc90197)