Organise the mfcgi modules into a library structure.

mfcgi.m:
    This is now the main module and it simply imports the other modules.

basic.m:
    The basic API has been moved here.

multi.m:
    Renamed this module.

mfcgi_test.m:
    Conform to above changes.

Makefile:
    Build the mfcgi code as a library.