cmFindPackageCommand: Fix RegEnumValueW data size computation

RegEnumValue returns ERROR_MORE_DATA if lpcchValueName is larger
than 32767 _bytes_, so reduce our name[] size to fit within that.
Also update the lpcbData argument and ERROR_MORE_DATA handling
to work with the size of the lpData buffer in bytes, not elements.
Merge topic 'unicode-win32-apis' into next

283c0a7 cmFindPackageCommand: Fix RegEnumValueW data size computation