The Windows HANDLE type, despite being a `void *', does not actually

The Windows HANDLE type, despite being a `void *', does not actually
behave like a pointer. In particular, the right thing to set a
HANDLE to to indicate that it's invalid is INVALID_HANDLE_VALUE, not
NULL. Crack down on sloppy use of NULL HANDLEs across all Windows
code.

(There is one oddity, which is that {Create,Open}FileMapping are
documented to return a NULL HANDLE instead of INVALID_HANDLE_VALUE
on failure. Shrug. If MS want to be inconsistent, I suppose I have
to live with it.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@6833 cda61777-01e9-0310-a592-d414129be87e