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/buffer_defs.h

@@ -356,6 +356,8 @@ struct mapblock {
   LuaRef m_luaref;         // lua function reference as rhs
   int m_keylen;                 // strlen(m_keys)
   int m_mode;                   // valid mode
+  int m_simplified;             // m_keys was simplified, do no use this map
+                                // if keys are typed
   int m_noremap;                // if non-zero no re-mapping for m_str
   char m_silent;                // <silent> used, don't echo commands
   char m_nowait;                // <nowait> used