core: add libgsystem as a git submodule

core: add libgsystem as a git submodule

And change src/main.c to use the local allocation macros.  This
results in much cleaner code, as one can see from the diff.

Because libgsystem is designed for nonrecursive make, it fits best in
the current recursive setup if we build . first.  This will be a lot
nicer when we switch NM to a nonrecursive setup.