Fix mail display when plaintext opt changes in run

* src/gpgoladdin.cpp (check_html_preferred): Export.
* src/gpgoladdin.h: Add prototype.
* src/mail.cpp (decryptVerify_o): check_html_preferred for each
decrypt.

This is the safest way and we only read a registry key
so this should not really decrease performance.

• GnuPG-Bug-Id: T4611