Resolve /dev/console to the active tty instead of just "tty0"

When resolving /dev/console one would often get "tty0" meaning the active VT.
Resolving to the actual tty (e.g. "tty1") will notably help on boot when
determining whether or not PID1 can output to the console.

src/shared/util.c

@@ -3600,6 +3600,16 @@ char *resolve_dev_console(char **active) {
         else
                 tty = *active;
 
+        if (streq(tty, "tty0")) {
+                char *tmp;
+
+                /* Get the active VC (e.g. tty1) */
+                if (read_one_line_file("/sys/class/tty/tty0/active", &tmp) >= 0) {
+                        free(*active);
+                        tty = *active = tmp;
+                }
+        }
+
         return tty;
 }