Merge from internal DataProtection repo.
Fix build break: inadvertently removed PBKDF2.cs and related functionality
Added support for protected data style DPAPI
- This won't work across core clr and desktop and mono but
 that's ok for the moment because it unblocks things
Update file headers
Updating build scripts
Updating build scripts
Create LICENSE.txt
Updating copyright headers
Create README.md
Create CONTRIBUTING.md
Initial CoreSystemServer fixup work