Make libusb-1.0 the default for USB support

When libusb-1.0 is not found, libusb-0.1 will be tried.  On BeOS and
OS2, nothing should have changed in terms of detection of USB support.

On all systems the --enable-libusb* flags are now ignored.  Instead, the
--with-usb and --without-usb flags now control support.  When neither is
given USB support will be enable if possible and disabled otherwise.  If
--with-usb is requested but not possible, ./configure will fail.  There
is no support to prefer libusb-0.1 over libusb-1.0.