Fix build on Linux & a compile warning.

-   force use of 'clang' on Linux
-   work around weird linking errors by doing away with remembyte.h
-   fix warning about mapping_t type
-   add a 'util' module. remembyte.c now contains *only* code for the cli
    tool... functions used by more than one module now belong in 'util.c'.
-   fix digittoint() under Linux by not inlining
-   fix segfault on Linux b/c 'int main()' wasn't returning anything
-   rename, add documentation, add comments for hit2int()
-   add a .gitignore file