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
vim/vim#3302) Also make emsg() accept a "char *" argument. Get rid of
an enormous number of type casts.
f9e3e09fdc
'name' param is casted to char_u* within get_option_value().
Most calls to get_option_value() cast arg to 'name' from char to char_u.
Remove these pointless type casts.
