Fix single character command line arguments.
Make dependence on '@' log files explicit.

barn will only ship log files beginning with '@', but no filtering for
those files was done until after rsync dry run. This change makes that
dependence more explicit and by doing so does more filtering. Also
removes bug that a directory with a non '@' file would probably cause
no shipping.
Add method to disable barn-agent in an emergency.

As barn-agent often runs under chef/puppet type environments it can be
difficult to disable a single instance. This change puts in an
emergency disable filename that operators can use to stop errant processes
from shipping.
Slight change to what gets logged to stdout.