terminal: always print to stderr with --no-input-terminal

The function terminal_in_background() reports whether the player was
backgrounded. In this case, we don't want to annoy the user by still
printing the status to stderr. If no terminal interaction is assumed,
this mechanism is disabled, and stderr is always used. The read_terminal
variable signals this case.

Oddly, just redirecting stderr will disable output to stderr, because
the background check with tcgetpgrp() is done on stderr, but
read_terminal is still true (because that one depends on stdin and
stdout).

Explicitly disable this mechanism if --no-input-terminal is used by
setting read_terminal to true only if terminal input is actually
initialized.

osdep/terminal-unix.c

@@ -416,6 +416,11 @@ void terminal_setup_getch(struct input_ctx *ictx)
     if (mp_make_wakeup_pipe(death_pipe) < 0)
         return;
 
+    // Disable reading from the terminal even if stdout is not a tty, to make
+    //   mpv ... | less
+    // do the right thing.
+    read_terminal = isatty(STDIN_FILENO) && isatty(STDOUT_FILENO);
+
     input_ctx = ictx;
 
     if (pthread_create(&input_thread, NULL, terminal_thread, NULL)) {
@@ -455,6 +460,7 @@ void terminal_uninit(void)
     }
 
     getch2_enabled = 0;
+    read_terminal = false;
 }
 
 bool terminal_in_background(void)
@@ -477,11 +483,6 @@ int terminal_init(void)
     assert(!getch2_enabled);
     getch2_enabled = 1;
 
-    // Disable reading from the terminal even if stdout is not a tty, to make
-    //   mpv ... | less
-    // do the right thing.
-    read_terminal = isatty(STDIN_FILENO) && isatty(STDOUT_FILENO);
-
     // handlers to fix terminal settings
     setsigaction(SIGCONT, continue_sighandler, 0, true);
     setsigaction(SIGTSTP, stop_sighandler, SA_RESETHAND, false);