Add new GEOM_PART_LDM module that implements the Logical Disk Manager

Add new GEOM_PART_LDM module that implements the Logical Disk Manager
scheme. The LDM is a logical volume manager for MS Windows NT and it
is also known as dynamic volumes. It supports about 2000 partitions
and also provides the capability for software RAID implementations.

This version implements only partitioning scheme capability and based
on the linux-ntfs project documentation and several publications across
the Web. NOTE: JBOD, RAID0 and RAID5 volumes aren't supported.

An access to the LDM metadata is read-only. When LDM is on the disk
partitioned with MBR we can also destroy metadata. For the GPT
partitioned disks destroy action is not supported.

Reviewed by:	ivoras (previous version)
MFC after:	1 month