pid1: disable color output when TERM=dumb

This changes the behaviour of pid1 in the following ways:
- obviously $TERM is now checked,
- $SYSTEMD_COLORS is now honoured too, before only SYSTEMD_LOG_COLORS was checked,
- isatty() is run on stdout not stderr.

As requested in #3025.

src/core/main.c

@@ -1338,7 +1338,7 @@ int main(int argc, char *argv[]) {
         saved_argv = argv;
         saved_argc = argc;
 
-        log_show_color(isatty(STDERR_FILENO) > 0);
+        log_show_color(colors_enabled());
         log_set_upgrade_syslog_to_journal(true);
 
         /* Disable the umask logic */