Fix warnings reported by -Wshadow: Part6: main directory

Now that all warnings are taken care of, add warning flag -Wshadow to
configure.

Note: Enabling this flag for C++ generates some useless warnings about
some function parameters shadowing class member function names. So, only
enabling this warning for C code.

Change-Id: I53ffb763483fec675a3358c5e64a737a03b93eb1