Issue warnings for potential switch w/o defining switch regions.

If either older style switches or new potential-switch modifiers are
used, issue a warning if the PME regions is too long, resulting in
inaccurate energies. Also issue a warning if rvdw_switch is 0, which
occurs if no value is specified.

Discussion is in redmine #1463

Change-Id: I7a2c29f87ceb04712aab5076ac97f5f22f573671