[plugin] implement global --all-logs option

Implement a global '--all-logs' option that causes all active
plugins to attempt to collect all logs present regardles of
rotation or size limiting. This replaces the numerous per-plugin
'all_logs' options and avoids the need for clumsy command lines
like:

 -k audit.all_logs,cron.all_logs,logs.all_logs,pcp.all_logs,...

The audit, cron, cups, libvirt, logs, mysql, and pcp plugins have
been converted to use the new option.

Fixes #305.

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