imgact_binmisc: Optionally pre-open the interpreter vnode

This allows the use of chroot and/or jail environments which depend on
interpreters registed with imgact_binmisc to use emulator binaries from
the host to emulate programs inside the chroot.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D37432

(cherry picked from commit 5eeb4f737f11b253ac330ae459b05e30fd16d0e8)