Try to convert clearsigned to utf8 on verify fail

* src/parsecontroller.cpp (ParseController::parse): Try again
with UTF-8 if verify opaque failed.

We have a bit of a problem in that we access the body
through the 8 bit interface and we need to give it
to GnuPG as 8 bit.

We now try utf8 if the native encoding did not
give us a result. It's a bit of a hack but will
work for messages from gpgol to gpgol and hopefully
for some others, too.

• GnuPG-Bug-Id: T3962