The selection character could be mistaken for part of another
keybinding.

This patch tries to solves this issue.  I tested it a bit with different
frame sizes and different fonts and get good enough results.

Here, how I justify the modification of `colwidth':

      - I don't think we need the "(min 30)" part since, if the frame is
        wide enough, we always get a colwidth of 30.
        
      - I don't think "(window-width)" is what we need since, by
        default, the *Completions* buffer will use the full frame width.

      - I tried to set the layout on three columns because "why not?"

While here, I had some clean up.

In GNU Emacs 31.0.50 (build 18, x86_64-unknown-openbsd7.6, X toolkit) of
 2024-10-09 built on computer
Repository revision: 9ed82c26a36caf9a9a85779cbb3a58b7ccd3dffb
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101013
System Description: OpenBSD computer 7.6 GENERIC.MP#344 amd64

Configured using:
 'configure CC=egcc CPPFLAGS=-I/usr/local/include
 LDFLAGS=-L/usr/local/lib MAKEINFO=gmakeinfo --prefix=/home/manuel/emacs
 --bindir=/home/manuel/bin --with-x-toolkit=lucid
 --with-toolkit-scroll-bars=no --without-cairo
 --without-compress-install'