fix setting up and using the terminfo data

There should be just one place that calls `setupterm()`. While refactoring
the code I also decided to not make initializing the curses subsystem a
fatal error. We now try two fallback terminal names ("ansi" and "dumb")
and if those can't be used we still end up with a usable shell.

Fixes #3850

src/fish_key_reader.cpp

@@ -294,7 +294,6 @@ static void setup_and_process_keys(bool continuous_mode) {
     setup_fork_guards();
     env_init();
     reader_init();
-    input_init();
     proc_push_interactive(1);
     install_our_signal_handlers();