Compatibility-related flexibilities.

- Added user-name and user-group overrides (DPT_ROOT_USER, DPT_ROOT_GROUP), for systems that don't have a 'root' group or when root isn't a desirable consideration when building packages.
- Can now disable hard-links (DPT_HARD_LINKS). The cp's "-l" parameter is not supported on Mavericks 10.9.2 .
- Replaced the "-sB" parameter with the "-sk" parameter in the "du" call. It's not supported on Mavericks 10.9.2 .
- Can now disable sudo (DPT_SUDO), since it's sometimes not desirable, and not required just to build the package (not in our Mac environment, at least).
- Added compatibility changes to doc.
Bugfix.
Merge pull request #2: Improve platform compat w/ environment variables