Add support for /dev/disk/by-{id,path,uuid,label}/* symlinks



This adds support for /dev/disk/by-{id,path,uuid,label}/* symlinks instead of plain /dev/sdX ones that often get reordered on boot.

Pull request might have some issues with coding standards of collectd (feel free to point them out and I'll fix them). That said I have been running this code on my server for few months without issues.

```
<Plugin disk>
        Disk "scsi-SATA_OCZ-VERTEX3_OCZ-...-part1"
        Disk "scsi-SATA_OCZ-VERTEX3_OCZ-...-part1"
        Disk "scsi-SATA_ST9500620NS..."
        Disk "scsi-SATA_ST9500620NS..."
        Disk "scsi-SATA_ST91000640NS..."
        Disk "scsi-SATA_ST91000640NS..."
        Disk "scsi-SATA_OCZ-VERTEX3_OCZ-...-part2"
        Disk "scsi-SATA_OCZ-VERTEX3_OCZ-...-part2"
        Disk "scsi-SATA_OCZ-VERTEX3_OCZ-...-part1"
        Disk "scsi-SATA_OCZ-VERTEX3_OCZ-...-part2"

        IgnoreSelected false
        UseNamedSymlinks true
</Plugin>
```