Update win32mbcs extension
* Code cleanup by Matt.
* Fix the issue with case-insensitive fs support
by wrapping also util.fspath() and util.checkcase()
* Abort program when path conversion is failed.
% should fail
abort: path contains illegal component: .hg/00changelog.i
adding a/a
% should fail
abort: path 'b/b' traverses symbolic link 'b'
% should succeed
% should still fail - maybe
abort: path 'b/b' traverses symbolic link 'b'