Improve ReadAsPlain detection

* src/common.cpp, src/common.h (read_reg_bool): New helper.
* src/gpgoladdin.cpp (check_html_preferred): Factor out
registry code and check also in policies.

This fixes the html preference detection in case the
preferences are set through the policies registry part and
also adds a fallback to HKLM options.

• GnuPG-Bug-Id: T5681