Problem: The return value of mode() does not indicate that completion is
active in Replace and Insert mode. (Zhen-Huan (Kenny) Hu)
Solution: Add "c" or "x" for two kinds of completion. (Yegappan Lakshmanan,
closes#1397) Test some more modes.
Problem: The 'langnoremap' option leads to double negatives. And it does
not work for the last character of a mapping.
Solution: Add 'langremap' with the opposite value. Keep 'langnoremap' for
backwards compatibility. Make it work for the last character of a
mapping. Make the test work.
Problem: Cannot map <M-">. (Stephen Riehm)
Solution: Solve the memory access problem in another way. (Dominique Pelle)
Allow for using <M-\"> in a string.
