Problem
=======

The mock-server can import javascript modules with require().

The module search path is controlled via "--module-prefix".

The option currently only allows to set one search path, which doesn't
allow to have modules in different directories (e.g. sourcedir +
binarydir)

Change
======

- allow to set --module-prefix multiple times
- convert duk_module_shim.c from C to C++
  - NULL -> nullptr
  - string.h -> cstring

RB: 25954