It varies on different versions of Windows, so we shouldn't test it for 
an exact value.

rpcrt4: Call IRpcChannelBuffer_GetDestCtx in NdrStubInitialize, just like we do in NdrProxyInitialize.

rpcrt4: Flesh out more of the IRpcChannelBuffer vtable to make the test_delegating_Invoke test succeed on Windows.

uninstaller: Iterate keys in HKEY_CURRENT_USER also as some programs install their uninstall information there.

winhelp: Correctly deal with various TopicBlockSize/Compression combinations. Simplify HLPFILE_Uncompress_Topic function.