The van der Waals radius must equal the Coulomb radius with Verlet

This patch fixes an issue that can occur when using g_tune_pme
with a Verlet pair-list .tpr input file. If the .tpr itself has
large cutoffs (e.g. 1.2 nm) and one asks g_tune_pme to scale
_down_ the Coulomb radius, the van der Waals radius is not
scaled down with the Coulomb radius (only upscaling worked).
One ends up with an unusable .tpr file because rVdW != rCoul.
This patch ensures that van der Waals and Coulomb radii
are always equal with Verlet pair-lists. Fixes #1460. Thanks
to Joao Rodrigues for reporting the issue!

Change-Id: I5ef30e71a35cd83838040057e16e52e09ea82e9a