Restructure the project

It was failing on 64-bit platforms due to insufficient configuration
in mlink and a51: DWORD was mapped to 64-bit long.

Added the necessary configuration and restructured the project:

- shared include files go under include/
- shared portability code is under lib/port and is compiled into a
  library
- rename src/ to a51/
examples/Makefile: a makefile to build the examples
lib/port: add strlwr to the library
Extract generic things from a51 into lib/common

Th emodules that were moved:
- clist.h
- misc
- debug.c
Added link51 license and copyright info
Import link51 source snapshot from 1998-02-11
Integrate link51 with the build
Add a modular example that requires real linking