CJK cleanups. Correct handling when the cursor is covering the

CJK cleanups. Correct handling when the cursor is covering the
right-hand half of a CJK wide character; correct handling of cut and
paste when CJK text wraps between lines _irrespective of the parity
of the starting column_; correct handling of wordness values
irrespective of which half of a CJK character the user
double-clicked on; correct handling when any terminal activity
overwrites only one half of a CJK wide character. I think we now
behave marginally better than xterm in this respect (it has a redraw
problem when you overwrite the RH half of a CJK char), so I'm happy.
Also redefined the internal UCSWIDE marker to something in the
surrogate range, while I'm here, so that U+303F is available for use
by actual users.

[originally from svn r2426]