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/reader.cpp

@@ -1539,10 +1539,10 @@ static bool check_for_orphaned_process(unsigned long loop_count, pid_t shell_pgi
 
 /// Initialize data for interactive use.
 static void reader_interactive_init() {
+    assert(input_initialized);
     // See if we are running interactively.
     pid_t shell_pgid;
 
-    input_init();
     kill_init();
     shell_pgid = getpgrp();