Allow no_hash to appear in manifest.

sbin/veriexec will ignore entries that have no hash anyway,
but loader needs to be explicitly told that such files are
ok to ignore (not verify).

We will report as Unverified depending on verbose level,
but with no reason - because we are not rejecting the file.

Reviewed by: imp, mindal_semihalf
Sponsored by: Juniper Networks
MFC After: 1 week
Differential Revision: https://reviews.freebsd.org//D20018