mac_veriexec: Authorize reads of secured sysctls

Writes to sysctls flagged with CTLFLAG_SECURE are blocked if the appropriate secure level is set. mac_veriexec does not behave this way, it blocks such sysctls in read-only mode as well.

This change aims to make mac_veriexec behave like secure levels, as it was meant by the original commit ed377cf41.

Reviewed by: sjg
Differential revision: https://reviews.freebsd.org/D34327
Obtained from: Stormshield