marsadm: print warnings when SPLIT BRAIN is detected
marsadm: disallow 'primary --force' in connected state

DRBD does not allow to deliberately provoke a SPLIT BRAIN
in connected state.

Thus MARS Light cannot be required to support such silly things
in any way.
marsadm: allow overrides of "marsadm secondary" in disconnected clusters

When the communication is down in partitioned clusters, the command
"marsadm seconodary" would normally use the current timestamp of
the command execution for the Lamport update protocol.

Make an exception for the case that a concurrent "marsadm primary"
was given somewhere else on a disconnected / partitioned node: let
the "primary" always win over a "secondary" command.
marsadm: wait for devices to {dis,}appear even on {prim,second}ary --force
marsadm: better error message at failed 'invalidate'
light: prevent logfile fetching on primaries and behind split brains

Save space in /mars/ by not fetching useless logfiles.