imgact_binmisc: limit the extent of match on incoming entries

imgact_binmisc matches magic/mask from imgp->image_header, which is only a
single page in size mapped from the first page of an image. One can specify
an interpreter that matches on, e.g., --offset 4096 --size 256 to read up to
256 bytes past the mapped first page.

The limitation is that we cannot specify a magic string that exceeds a
single page, and we can't allow offset + size to exceed a single page
either. A static assert has been added in case someone finds it useful to
try and expand the size, but it does seem a little unlikely.

While this looks kind of exploitable at a sideways squinty-glance, there are
a couple of mitigating factors:

1.) imgact_binmisc is not enabled by default,
2.) entries may only be added by the superuser,
3.) trying to exploit this information to read what's mapped past the end

would be worse than a root canal or some other relatably painful
experience, and

4.) there's no way one could pull this off without it being completely

obvious.

The first page is mapped out of an sf_buf, the implementation of which (or
lack thereof) depends on your platform.

MFC after: 1 week