main: don't mount /sys, /dev and friends when we run with PID != 1

Back from old times when we developed systemd on non-systemd hosts we
still mounted the missing directories such as the cgroup stuff even when
running with a PID != 1. There's no point for that anymore, so let's
just do that if we are actually PID 1, and never otherwise.

https://bugs.freedesktop.org/show_bug.cgi?id=62354

src/core/main.c

@@ -1437,7 +1437,7 @@ int main(int argc, char *argv[]) {
 
         /* Mount /proc, /sys and friends, so that /proc/cmdline and
          * /proc/$PID/fd is available. */
-        if (geteuid() == 0 && !getenv("SYSTEMD_SKIP_API_MOUNTS")) {
+        if (getpid() == 1) {
                 r = mount_setup(loaded_policy);
                 if (r < 0)
                         goto finish;