Add mechanism to specify named win32 NT handles.

[nikhiljnv]

Currently, cl_khr_external_semaphore and cl_khr_external_memory define properties to specify external win32 NT handle via pointer. Win32 NT handles can also be specified via named strings.

Add properties to specify external Win32 NT handles via named strings too.

Fixes #943

[oddhack]

@bashbaug @nikhiljnv with #950 merged, this will have to be rewritten in terms of the new API spec structure and markup. I'm not sure how many more outstanding PRs that modify or add new khr extensions there are but they will all be likewise affected.

I can assist if need be, but this may act as a good test of how difficult it will be for people to move over to the new model.

[nikhiljnv]

Closing in favor of more recent PR #1177