VS: Use lower-case boolean values in VS 7-9 (#14927)

The VS 7-9 IDEs parse .vcproj file boolean values in lower or upper
case.  The .NET XML parsing chokes on anything but "true", "false", "0",
"1".  Teach our generators to use lower-case names since they will work
for both parsers.  Our VS >= 10 flag tables already use lower-case.
Merge topic 'vs-vcproj-bool-case' into next

b684ce58 VS: Use lower-case boolean values in VS 7-9 (#14927)