ui: Remove/adapt some old code for a big UI refactor

- Remove abstract_ui global, now it is always active
- Remove some terminal handling code
- Remove unused functions
- Remove HAVE_TGETENT/TERMINFO/TERMIOS/IOCTL #ifdefs
- Remove tgetent/terminfo from version.c
- Remove curses/terminfo dependencies
- Only start/stop termcap when starting/exiting the program
- msg_use_printf will return true if there are no attached UIs(
  messages will be written to stdout)
- Remove `ex_winpos`(implement `:winpos` with `ex_ni`)

src/nvim/keymap.c

@@ -752,26 +752,3 @@ int get_mouse_button(int code, bool *is_click, bool *is_drag)
     }
   return 0;         /* Shouldn't get here */
 }
-
-/*
- * Return the appropriate pseudo mouse event token (KE_LEFTMOUSE etc) based on
- * the given information about which mouse button is down, and whether the
- * mouse was clicked, dragged or released.
- */
-int 
-get_pseudo_mouse_code (
-    int button,             /* eg MOUSE_LEFT */
-    int is_click,
-    int is_drag
-)
-{
-  int i;
-
-  for (i = 0; mouse_table[i].pseudo_code; i++)
-    if (button == mouse_table[i].button
-        && is_click == mouse_table[i].is_click
-        && is_drag == mouse_table[i].is_drag) {
-      return mouse_table[i].pseudo_code;
-    }
-  return (int)KE_IGNORE;            /* not recognized, ignore it */
-}