Dear Fortranners,

the attached patch is a follow-up to the fix for PR107441,
as it finally fixes the treatment of character dummy arguments
that have the value,optional attribute, and allows for checking
of the presence of such arguments.

This entails a small ABI clarification, as the previous text
was not really clear on the argument passing conventions,
and the previously generated code was inconsistent at best,
or rather wrong, for this kind of procedure arguments.
(E.g. the number of passed arguments was varying...)

Testcase cross-checked with NAG 7.1.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald