readline: Assume "unknown" terminal type by default

This patch modifies the `_rl_init_terminal_io` function to pass
`unknown` terminal type instead of `dumb` when no terminal name is
provided by the caller (e.g. when the `TERM` environment variable is
not set on Win32).

This ensures that the termcap provider (e.g. ncurses) resolves the
default preferred terminal type instead of using the `dumb` terminal
type.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Author: stephanosio

readline/readline/terminal.c

@@ -478,7 +478,7 @@ _rl_init_terminal_io (const char *terminal_name)
   tty = rl_instream ? fileno (rl_instream) : 0;
 
   if (term == 0)
-    term = "dumb";
+    term = "unknown";
 
   dumbterm = STREQ (term, "dumb");