Set ISOPEN in namei flags when opening executable interpreters.

These vnodes are explicitly opened via VOP_OPEN via
exec_check_permissions identical to the main exectuable image.
Setting ISOPEN allows filesystems to perform suitable checks in
VOP_LOOKUP (e.g. close-to-open consistency in the NFS client).

Reviewed by: kib
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D21129