Allow printing potential energies to dhdl.xvg file

Previously, only total energies were printed out to the dhdl.xvg
file.  In many cases, it is more useful to print out just
the potential energies.  For example, when doing mdrun -rerun,
and comparing to output run without rerun, the velocities may not
be identical, which makes it difficult to compare the total
energies between runs when computing free energies and other
observables. dhdl-print-energy can now be 'no', 'total',
or 'potential'. The alternative 'kinetic' should generally not
be needed, since that can be done analytically in all cases.

Backwards compatible in the .tpr since gmx_bool is an int. The
old false value will be interpreted as 'no', and the 'yes' value
as 'total' energy, which will reproduce the old behavior.

Fixes #1329

Change-Id: I16828ef07c46bcfc61fe03744cbf251c00160636