vim-patch:8.1.2145: cannot map <C-H> when modifyOtherKeys is enabled

Problem:    Cannot map <C-H> when modifyOtherKeys is enabled.
Solution:   Add the <C-H> mapping twice, both with modifier and as 0x08.  Use
            only the first one when modifyOtherKeys has been detected.
459fd785e4

Add REPTERM_NO_SPECIAL instead of REPTERM_SPECIAL because the meaning of
"special" is different between Vim and Nvim.
Omit seenModifyOtherKeys as Nvim supports attaching multiple UIs.
Omit tests as they send terminal codes.
Keep the behavior of API functions.

src/nvim/ex_docmd.c

@@ -5180,8 +5180,7 @@ int uc_add_command(char_u *name, size_t name_len, char_u *rep, uint32_t argt, lo
   char_u *rep_buf = NULL;
   garray_T *gap;
 
-  replace_termcodes(rep, STRLEN(rep), &rep_buf, false, false, true,
-                    CPO_TO_CPO_FLAGS);
+  replace_termcodes(rep, STRLEN(rep), &rep_buf, 0, NULL, CPO_TO_CPO_FLAGS);
   if (rep_buf == NULL) {
     // Can't replace termcodes - try using the string as is
     rep_buf = vim_strsave(rep);