[NTFS] - Commit early results of a small restructuring effort:
-Add a new member to the NTFS_ATTR_CONTEXT struct, a LARGE_MCB. This allows an attribute context to describe the cluster mapping of a non-resident file while allowing that mapping to change dynamically, without the context itself needing to be resized. This fixes problems which sometimes arose from resizing files.
-Remove hacky code from NtfsWriteFile() for dealing with "stale" contexts. This fixes that issue.
-Update SetDataAttributeLength(), PrepareAttributeContext(), ReleaseAttributeContext(), FreeClusters(), and AddRun() for the new member.
-Update ReadAttribute() and WriteAttribute() to work with the changed structure. A very-soon-to-come commit will overhaul these functions so they'll operate directly on the LARGE_MCB, instead of converting to and from a packed list of data runs. (Sparse files are broken until then.)
-Rename "RunBufferOffset" to "RunBufferSize" in several places where appropriate.
-Fix, improve, and add some comments.
svn path=/branches/GSoC_2016/NTFS/; revision=74523