Make init_dires independent of t_commrec

init_disres was requesting a full pointer to the commrec, but only uses a single communicator and checks for master rank and whether the run is parallel. This information is now passed in explicitly, simplifying the planned splitting of t_commrec.

Note that passing a nullptr for commrec was (mis)used by gmx_disre only - effectively signalling that init_disres was called from an analysis tool and not from mdrun. This has been made explicit.

Refs #2395