keep a reference to the device for each active transfer and let the backend handle cancelling active transfers when a device is disconnected

This commit should fix issues with active transfers when a device is
disconnected. The backend is responsible for making sure the completion
callbacks are made, not the hotplug code. This should fix a number of
issues including duplicate callbacks and segmentation faults.

References #124.
Add checks for headers needed by linux/netlink.h

These headers are required by netlink.h. I am not sure how this
worked at all. It certainly doesn't work with older versions of Linux
2.6.

References #124.
Silence automake 1.14 warning

The warning is:

libusb/Makefile.am:57: warning: source file 'os/threads_windows.c' is in a subdirectory,
libusb/Makefile.am:57: but option 'subdir-objects' is disabled
automake: warning: possible forward-incompatibility.
automake: At least a source file is in a subdirectory, but the 'subdir-objects'
automake: automake option hasn't been enabled.  For now, the corresponding output
automake: object file(s) will be placed in the top-level directory.  However,
automake: this behaviour will change in future Automake versions: they will
automake: unconditionally cause object files to be placed in the same subdirectory
automake: of the corresponding sources.
automake: You are advised to start using 'subdir-objects' option throughout your
automake: project, to avoid future incompatibilities.

Fixed by setting the subdir-objects option.

Closes #125.
include stdlib.h for free and realloc in libusbi.h

References #124.
fix warnings in linux_get_device_address

Warnings:

os/linux_usbfs.c: In function 'linux_get_device_address':
os/linux_usbfs.c:620: warning: comparison is always false due to limited range of data type
os/linux_usbfs.c:624: warning: comparison is always false due to limited range of data type
os/linux_usbfs.c:628: warning: comparison is always false due to limited range of data type
os/linux_usbfs.c:628: warning: comparison is always false due to limited range of data type
work around Linux systems that don't provide SOCK_CLOEXEC or SOCK_NONBLOCK

These options were added in 2.6.27 and are not available on all kernels
that support netlink. Set these options using fcntl when SOCK_CLOEXEC and
SOCK_NONBLOCK are not available.

Closes #124.