Use SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE flag for symlink functions

[darealshinji]

Shouldn't the symlink() functions in MsvcLibX use the SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE flag?
I always need to add this to the flags of CreateSymbolicLink to create links as normal user.

[JFLarvoire]

Sounds like a good idea. Please go ahead and submit a pull request!
Also we must add a definition for the flag if needed, so that the library still builds for XP and older versions of Windows, and apps built with it use the flag when run in recent versions of Windows that support it.