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/vim.h

@@ -318,4 +318,10 @@ enum { FOLD_TEXT_LEN = 51, };  //!< buffer size for get_foldtext()
 #define REPLACE_CR_NCHAR    (-1)
 #define REPLACE_NL_NCHAR    (-2)
 
+// Flags for replace_termcodes()
+#define REPTERM_FROM_PART       1
+#define REPTERM_DO_LT           2
+#define REPTERM_NO_SPECIAL      4
+#define REPTERM_NO_SIMPLIFY     8
+
 #endif  // NVIM_VIM_H