I commited a few minor changes to musbstd and mscb code to make them work on
MacOSX (tested on 10.3.9) and Linux (tested on Fedora 6).

The basic functions work with the MSCB USB master, but I still need to
investigate some cases where the connection hangs and usb communications do not
work until the USB cable is unplugged and plugged back in. I see this problem
both on MacOS and Linux.

Important changes:
1) mscb_select_device() does not work on both Linux and MacOS and is disabled.
Please run "msc -d usb0".
2) on Linux, the Makefile should define -DOS_LINUX and -DHAVE_LIBUSB;
   on MacOS, the Makefile should define -DOS_LINUX and -DOS_DARWIN. (This is
because MacOS is treated as a funny type of Linux).
3) when doing USB communications, one has to use the correct endpoint numbers,
which seem to be system dependant and for now, I hard code them in mscb.c for
the tested systems.

There supposed to be no changes to the Windows code, but I cannot test on
Windows, so if somebody does and finds breakage, please let me know.

K.O.