Revert to treating -1 as "generate a seed" in .mdp
Commit
2d0247f6 changed the grompp behaviour while refactoring the rng
seed setup, without mentioning it in the docs or commit message.
Several of the tools also had their functionality changed, but in most
cases there was no previous documentation and now there is
some. However, we can't change grompp behaviour without catering for
all the scripts whose behaviour would change.
This change restores the previous functionality for grompp.
Change-Id: Id77098e1f03ebf96207f8c9908b2dada28c59e6a