linux: Mark internal file descriptors as CLOEXEC

linux: Mark internal file descriptors as CLOEXEC

As a library, libusb should take care to be as friendly as possible
with various use cases. One such way is to ensure that internal file
descriptors have the CLOEXEC flag set, thus allowing processes to do
a fork() + exec() without leaking libusb's file descriptors to the
child process.

References #268

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>