usbi_handle_transfer_completion: Remove signal to event waiters

usbi_handle_transfer_completion() is only called by backends when
handling transfer completion. The backend can only make this call
whilst holding the events lock, therefore it is pointless to send
this signal. Any threads waiting to be signaled will be woken up
and will either find that an event handler is still active or will
try to obtain the events lock and be blocked.

Event waiters are automatically signaled in libusb_unlock_events(),
so when the backend is done handling events it will release the lock
and wake up any waiters. At this point the events lock wll be free,
which is the only time waking up waiters makes sense.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
core: Only interrupt event handlers as necessary during libusb_open()

The current behavior of libusb_open() unconditionally interrupts
any event handlers via usbi_fd_notification(). However, not all
backends, namely Win/WinCE, make changes to the pollfd list during
a device open.

This change adds a new CAP for backends (HAS_POLLABLE_DEVICE_FD),
and libusb_open() will only call usbi_fd_notification() if the
backend declares this capability.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
core: Be more efficient by not passing internal fds to backend

There are a number of fds that the core uses internally. Currently
any events on these fds are masked out so that the backend will
skip over them during handle_events().

This change improves upon this by simply not providing these fds
to the backend.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
Misc: Add myself to the README as a maintainer
core: Reuse poll fds across calls to handle_events()

Prior to this patch, the array of poll fds given to poll() was
allocated and freed every time handle_events() was called. This
is unnecessary if the list of poll fds has not changed since the
last call to handle_events(). With this patch, the array and count
of poll fds is stored in the context and only reallocated when the
list of poll fds changes, saving any unnecessary overhead.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
core: Rename count variable for pollfds for consistency with other count variables

Signed-off-by: Chris Dickens <chris.dickens@hp.com>
core: Fix reference count leak for default context

If the default context was created but failed to fully initialize,
the reference count was not decremented appropriately.
Windows: Define WINVER to fix building on MinGW

* For newer versions of MinGW, VerSetConditionMask() is only defined
  if building for Windows XP (0x501) or newer.
core: Fix compiler warnings on MinGW and Visual Studio

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
core: Make use of headers consistent across all files

* Include config.h before anything else in all files
* Remove unnecessary inclusion of libusb.h
* Use angle brackets for system headers and quotes for local headers

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
threads_posix: Fix compiler warning due to redefinition of _GNU_SOURCE

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
io: Fix memory leaks in usbi_io_init()

* If the hotplug pipe failed to be created, the usbi_pollfd for the
  control pipe was being leaked.
* If the usbi_pollfd for timerfd failed to be added, the usbi_pollfd
  for the hotplug pipe was being leaked.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
io: Move timerfd close into error handling path

This makes error handling consistent with other areas. This is a
follow-on change to commit b72f4cf77edb1975cada8aab5ae63361c68c5992.

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