reg.exe: Add path_get_key() to remove boilerplate
reg.exe: Add wchar/type conversion functions
reg.exe: Add wchar/raw data conversion functions
reg.exe: Implement query functionality
reg.exe: Clean up reg_add

This enables certain edge cases such as
* reg add HKCU\\a /v b /d c
* reg add HKCU\\a\\b\\c\\d