As a follow-up issue from #3719, it was noticed that dynamic calls to
built-in SUBs did not work.

This issue was caused by more (struct symbol) members missing
initialization in VCC_New().

In turn, it became apparent that the evaluation context check in
vcc_expr5() as tested in v00020.vtc only worked by accident, and only
for built-in subs because (struct symbol).eval was NULL for built-in
subs.

We fix SUB type expression evaluation with a type specific evaluation
function which allows expression evaluation from explicit SUB contexts
only.

Fixes #3720