Integrate link51 with the build
Add a modular example that requires real linking
Fix the DOS build.
README.asc: update build instructions and examples info
mlink: BUGFIX: 11-bit fixups weren't validated correctly

There were missing parenthesis around "&" and the check
for 11-bit range succeeded where it should have  failed.

Fixed the check and added slightly better error diagnostics.
Print errors to stderr instead of stdout
link51: use the error reporting functions for unresolved symbols

link51 was using printf() until now.
link51: better error diagnostics for invalid 11-bit fixup