Restore honoring --max-c-ident-len for PGI.

The compiler was not honoring --max-c-ident-len aka fMaxCIdentLen,
which is set automatically for PGI compilation. As a result,
too-long identifiers got codegen-ed and PGI complained.

The culprit I think was the addition of CLASS_STRUCT_PREFIX and "_s"
(or, possibly, "_object") in AggregateType::classStructName().

Now these additions are accounted for via maxCNameAddedChars.

I do not know how to set maxCNameAddedChars automatically,
so I overshot it by a little and added reminder comments to
AggregateType::classStructName().