Problem: Giving error messages is not flexible.
Solution: Add semsg(). Change argument from "char_u *" to "char *", also
for msg() and get rid of most MSG macros. (Ozaki Kiichi, closes
#3302) Also make emsg() accept a "char *" argument. Get rid of
an enormous number of type casts.
Problem: Encryption implementation is messy. Blowfish encryption has a
weakness.
Solution: Refactor the encryption, store the state in an allocated struct
instead of using a save/restore mechanism. Introduce the
"blowfish2" method, which does not have the weakness and encrypts
the whole undo file. (largely by David Leadbeater)
