[plugin] implement global --log-size option

Implement a global '--log-size' option that causes all plugins
implementing log size limiting (via add_copy_spec_limit()) to
apply the specified value as the size limit.

Convert the auditd, cups, logs, nscd, rabbitmq, squid, and
upstart plugins to use the new option.

Fixes #306.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>