Windows/WinCE: Improve the run-time dynamic linking scheme

Windows/WinCE: Improve the run-time dynamic linking scheme

Prior to this commit, the Windows and WinCE backends had a specific
shortcoming when loading DLLs, that being that once loaded they were
never unloaded. This commit improves this by providing a means to
unload the DLLs during cleanup.

Note that the use of GetModuleHandle() has been removed in favor of
the exclusive use of LoadLibrary(). This was done to ensure that a
reference count is taken against the loaded DLL, which guards against
some other part of the application unloading the DLL that libusb
is currently using.

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