Require that external functions have explicit external call

[henrikt-ma]

This addresses the problem that we didn't specify any difference in semantics of what this means in Base Modelica:

pure external function 'L.P.f'
  input Real 'u';
  output Real 'y';
external "C";
end 'L.P.f';

Clearly, the Modelica semantics makes no sense, since 'L.P.f' is not a valid identifier in C.

The PR proposes to go with the first of the two possible solutions that come to mind:

• Don't allow leaving out the external-function-call.
• Specify a different way of deriving the default name of the C function, by requiring that the external function's name is a structured Base Modelica identifier, where it is the last identifier of the unmangled name that gives the name of the C function.

[perost]

I agree in principle, but it seems something's gone wrong with the PR since it includes hundreds of old commits.

[henrikt-ma]

I agree in principle, but it seems something's gone wrong with the PR since it includes hundreds of old commits.

Wrong base. Thanks for pointing it out!