This has been spotted and solved manually several times already,
and that's just boring so let's add the ability to state that
X11-based software is not accepted into a particular rescue image.

Not that I would hate X but things like that belong to a carefully
crafted image which includes either X server or reasonable means
to ensure that GUI software can actually be used.

NB: this is a somewhat new entity: test/rescue/no-x11 knob
for an image-script intended to make it blow up the build
when libX11 is found within the chroot that makes up
the rescue image's filesystem.

The interface is not documented intentionally: it will take
some time to find out whether it sticks or is bad enough.
Please do remind/ask if interested in using that.