Linux patches for 1.0.9 release

[Hans de Goede]

Hi All,

As discussed I would like to see these 6 patches added to the
libusbx tree for the 1.0.9 release.

The first 2 have already been re-viewed and ack-ed
by Peter, but for some reason not yet applied to his tree.

The next 2 fix error handling under Linux:
1) Fix the not handling of certain valid error codes under Linux
  (causing an error to be printed and a generic error code to be returned).
2) Fix linux error codes being passed unmodified as libusb transfer
   status codes for iso transfers

The last 2 are just small (and safe) cleanups, which could be deferred
to past 1.0.9 if people prefer.

Regards,

Hans