input: add support for modifyOtherKeys=2

Similar to modifyOtherKeys=1 (foot’s default, and only, mode), except
that:

* All modifiers (and not just Ctrl) generate \E[27;m;n~ escapes
* Regular keys (with modifiers) also generate \E[27;m;n~ escapes (for
  example, C-h no longer generates ^H, but \E[27;5;104~)

For our keymap based lookups, this is handled by adding
MOD_MODIFY_OTHER_KEYS_STATE<N> variants.

For “generic” keys, we simply adjust the conditions for when to emit a
\E[27;m;n~ escape - the only requirement is that at least one modifier
is active.

terminal.h

@@ -308,6 +308,7 @@ struct terminal {
     bool focus_events;
     bool alt_scrolling;
     bool modify_escape_key;
+    bool modify_other_keys_2;  /* True when modifyOtherKeys=2 (i.e. “CSI >4;2m”) */
     enum cursor_origin origin;
     enum cursor_keys cursor_keys_mode;
     enum keypad_keys keypad_keys_mode;