Security attributes on mmap file

This collection of changesets fixes #98 by setting security attributes on the memory-mapped file when communicating with the Pageant process in very much the same way as the reference implementation in PuTTY. In order to implement this fix in a pure-python, portable way, I've used extensive use of ctypes, and implemented in jaraco.windows. In order to avoid the dependency on jaraco.windows, the last changeset removes that dependency by inlining the code from the required modules. If you would prefer to keep jaraco.windows as an external dependency, you may omit that from the pull.

I'm eager to get this fix into the next release. Please let me know if there's anything more I can do to help.