More control over file backups

Add a gmx_set_max_backup_count() to specify how many backups should be
made, instead of just relying on the environment variable.  Make the
default to be no backups: now tests and FAHCORE do not need special
treatment.  Instead, make the command-line initialization specify the
maximum backup count, and add a command-line option that also allows
disabling backups as an alternative to the environment variable.

This also allows removing a valgrind suppression for memory leaks from
the test code.

Change-Id: Ib512eb2a491691c76404aacbf93b67087a1afe41