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_cmds2.c

@@ -155,10 +155,6 @@ void do_debug(char_u *cmd)
 #define CMD_INTERRUPT   6
 
 
-  /* Make sure we are in raw mode and start termcap mode.  Might have side
-   * effects... */
-  starttermcap();
-
   ++RedrawingDisabled;          /* don't redisplay the window */
   ++no_wait_return;             /* don't wait for return */
   did_emsg = FALSE;             /* don't use error from debugged stuff */