kern: imgact: fix imgp->interpreted

This is a mask, so the new value should have taken the next bit to avoid
breaking a shell script that's interpreted by a binmisc-activated
interpreter.

Add a brief note that the new value is only used within the ELF
activator.

Fixes: 389c124fecb0 ("imgact_elf.c indicate that interpreter [...]")
Reported by: "polyduekes" on discord, madpilot
Reviewed by: kib, sjg (both previous version)
Differential Revision: https://reviews.freebsd.org/D58063