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>
Merge branch 'master' of https://github.com/libusb/libusb

Conflicts:
	libusb/libusbi.h