Core: Fix writing of log lines a single fprintf call

* fb4c208c33788068bbca67bdd6d11127b5be5a26 broke cygwin compilation due to __GCC__ not being defined
* The actual issue is that __GNUC__ rather than __GCC__ should have been used all along
* Also fixes gettimeofday() usage for MinGW/Cygwin
* Also increase log buffer size to 1K, fix a broken fprintf in core.c and sort whitespaces
Core: Use fputs(3) instead of fprintf(3)

This is a micro-optimisation, but it should make the code easier to understand.
Prepare for 1.0.16-rc2 release

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Doc: update hotplug, topology and descriptor documentation

* Also fix some typos
* Closes #95
hotplug: Give the usbi_hotplug_match* functions a context parameter

So that the device parameter can be NULL, in combination with a 0
events parameter, to be used to force lazy deregistration.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
hotplug: Wakeup libusb_handle_events on libusb_hotplug_deregister_callback

This serves 2 purposes:

1) We use lazy free-ing of the callback structure, for it to be actually
free-ed usbi_hotplug_match() needs to be called. This ensures this actually
happens (rather then waiting for a hotplug event to arrive, and not freeing
the callback as long as no such event arrives).

2) It causes libusb_handle_events to return to its caller on a call to
libusb_hotplug_deregister_callback, which is very useful for apps which use
a thread to do their apps (hotplug) event handling, otherwise that thread will
hang when the app tries to stop until some event happens.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Documentation: explain the 2 main viable event handling approaches

Stop pretending that having a separate event handling thread is a bad thing,
specifically delete the "[this] option is not very nice either, but may be
the nicest option available to you if the "proper" approach can not be applied
to your application", which suggests that using poll integration into a main
loop is the one and only "proper" approach.

Instead clearly document there are 2 viable approaches, using a separate
thread, or poll integration into a main loop.

Also stop claiming that libusb does not use threads internally, as with
the new hotplug support this is no longer true.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Documentation: add an Using an event handling thread section

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Prepare for 1.0.16-rc3 release

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Prepare for 1.0.16 final release

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
hotplug: Pass explicit context to callbacks

* Instead of passing NULL for the context to hotplug callbacks, if the
  context happens to be the default context, always pass the explicit
  context pointer.
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.
linux: Use a separate lock to serialize start/stop vs hotplug events

Using one lock for this is a bad idea, as we should not be holding any
locks used by the hotplug thread when trying to stop otherwise the stop
function may wait indefinetely in pthread_join, while the event-thread
is waiting for the lock the caller of the stop function holds.

Using 2 separate locks for this should fix this deadlock, which has been
reported here: https://bugzilla.redhat.com/show_bug.cgi?id=985484

Many thanks to Chris Dickens for figuring out the cause of this deadlock!

CC: Chris Dickens <christopher.a.dickens@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
hotplug: Remove use of pthread_cancel from linux_udev

Using pthread_cancel() presents the opportunity for deadlock, so
use a control pipe to cause the event thread to gracefully exit.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
linux_netlink: close netlink socket on init error

Signed-off-by: Hans de Goede <hdegoede@redhat.com>