patch 8.1.0743: giving error messages is not flexible

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.
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 6bb930c..a419a05 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -1799,7 +1799,7 @@
 #ifdef FEAT_USR_CMDS
     int		useridx;	/* user command index */
 #endif
-    char_u	*errmsg;	/* returned error message */
+    char	*errmsg;	/* returned error message */
     char_u	*(*getline)(int, void *, int);
     void	*cookie;	/* argument for getline() */
 #ifdef FEAT_EVAL