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/strings.c

@@ -300,17 +300,6 @@ void del_trailing_spaces(char_u *ptr)
     *q = NUL;
 }
 
-/*
- * Like strncpy(), but always terminate the result with one NUL.
- * "to" must be "len + 1" long!
- */
-void vim_strncpy(char_u *restrict to, const char_u *restrict from, size_t len)
-  FUNC_ATTR_NONNULL_ALL
-{
-  STRNCPY(to, from, len);
-  to[len] = NUL;
-}
-
 /*
  * Like strcat(), but make sure the result fits in "tosize" bytes and is
  * always NUL terminated.