Add support for named symlinks for linux for disk plugin

This adds support for /dev/disk/by-{id,path,uuid,label}/* symlinks instead of plain /dev/sdX ones that often get reordered on boot.

Since my C is VERY rusty I'd appreciate if someone reviewed this pull request. It also might have some issues with coding standards of collectd (feel free to point them out and I'll fix them).

Thanks.