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/ex_cmds.lua

@@ -2483,7 +2483,7 @@ return {
   {
     command='winpos',
     flags=bit.bor(EXTRA, TRLBAR, CMDWIN),
-    func='ex_winpos',
+    func='ex_ni',
   },
   {
     command='wnext',