linux_udev: Don't free the udev_monitor while the event thread may still use it

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
POSIX: Set usbi_pipe to non-blocking by oring O_NONBLOCK to fd flags.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Merge branch 'master' of https://github.com/libusbx/libusbx