patch 7.4.1229

Problem:    "eval" and "expr" channel commands don't work yet.
Solution:   Implement them.  Update the error numbers.  Also add "redraw".

src/proto/json.pro

@@ -1,4 +1,5 @@
 /* json.c */
 char_u *json_encode(typval_T *val);
+char_u *json_encode_nr_expr(int nr, typval_T *val);
 void json_decode(js_read_T *reader, typval_T *res);
 /* vim: set ft=c : */