Verify files loaded in chain command.

The chain command can be used to chain load another binary.
If veriexec is enabled we should verify it first.
Note that on EFI systems the verification was already done
through firmware, assuming that Secure Boot was enabled there.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: sjg
MFC after: 1 week
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D20952