Add HKEY_USERS to reg.py and correct HKU hive mapping

[tsujamin]

Adds HKEY_USERS hive support to reg.py as "HKU", and renames access to HKEY_CURRENT_USER from "HKU" to "HKCU" to more closely match convention.

While the renaming of the prior HKU to HKCU is technically correct, if the preference is to not change existing behaviour I'm happy to revert that change and map HKEY_USERS to something else ("HKUSERS"?)

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[tsujamin]

Any changes needed?