feature: add support for FALLOC_FL_UNSHARE_RANGE

[dlrobertson]

We do not currently support FALLOC_FL_UNSHARE_RANGE. Adding support for it could be an interesting project. The following is from fallocate(2):

If the FALLOC_FL_UNSHARE_RANGE flag is specified in mode, shared file data extents will be made private to the file to guarantee that a subsequent write will not fail due to lack of space. Typically, this will be done by performing a copy-on-write operation on all shared data in the file. This flag may not be supported by all filesystems.

[koverstreet]

We definitely want this