ask-password: Add --echo to enable echoing the user input

Programs such as OpenVPN may use ask-password for not only retrieving
passwords, but also usernames.  Masking usernames with * seems just silly.

 v2 - Don't mess with termios flags, instead print the input
      instead of an asterix.  Resolves issues with backspace
      and TAB input.

 v3 - Renamed 'do_echo' variables and argument to 'echo'.  Also
      modified the ask_password_{tty,agent,auto} API instead of
      additional wrapper functions.

[zj: undo changes to ask_password_auto, since no callers were using
     the new argument.]

src/firstboot/firstboot.c

@@ -491,7 +491,7 @@ static int prompt_root_password(void) {
         for (;;) {
                 _cleanup_free_ char *a = NULL, *b = NULL;
 
-                r = ask_password_tty(msg1, 0, NULL, &a);
+                r = ask_password_tty(msg1, 0, false, NULL, &a);
                 if (r < 0) {
                         log_error("Failed to query root password: %s", strerror(-r));
                         return r;
@@ -502,7 +502,7 @@ static int prompt_root_password(void) {
                         break;
                 }
 
-                r = ask_password_tty(msg2, 0, NULL, &b);
+                r = ask_password_tty(msg2, 0, false, NULL, &b);
                 if (r < 0) {
                         log_error("Failed to query root password: %s", strerror(-r));
                         clear_string(a);