Add working example code to provide a starting point for users with files
that they can just copy, and include the modules in the coverage test to
verify the complete user C module build functionality. The cexample module
uses the code originally found in cmodules.rst, which has been updated to
reflect this and partially rewritten with more complete information.
Support C++ code in .cpp files by providing CXX counterparts of the
_USERMOD_ flags we have for C already. This merely enables the Makefile of
user C modules to use variables specific to C++ compilation, it is still up
to each port's main Makefile to also include these in the build.
