mp_msg: print MSGL_STATUS to stderr if stderr is redirected

Of course, status output is still hidden when stderr is a tty which is
not the foreground tty.

core/mp_msg.c

@@ -111,8 +111,8 @@ int mp_msg_test(int mod, int lev)
 {
 #ifndef __MINGW32__
     if (lev == MSGL_STATUS) {
-        // skip status line output if stderr is not in the fg process group
-        if (tcgetpgrp(2) != getpgrp())
+        // skip status line output if stderr is a tty but in background
+        if (isatty(2) && tcgetpgrp(2) != getpgrp())
             return false;
     }
 #endif