Create sendmail_mailq plugin

This plugin is a replacement for sendmail_mailqueue.

It's proposed at the contrib tree but it may eventually replace the main sendmail_mailqueue plugin.

It solves three problems in sendmail_mailqueue :

1. sendmail_mailqueue gets its results from the number of files on the queues directories. This may led to wrong results on some situations. sendmail_mailq parses the results of the command mailq, which is how sendmail identifies the contents of its queues.

2. It show separate results for the MTA and MSP.

3. In huge servers it's usually better to create multiple queues (or groups) and to set different priority to them, depending on the source or destination. This plugin shows the details for each queue.