Check inode number to see if we are in init namespace (#35306)

This is a more comprehensive fix compared to #35273. Also adds a minimal
test only.

Based on Luca's #35273 but generalizes the code a bit.

In v258 we really should get rid of the old heuristics around userns and
cgroupns detection, but given we are late in the v257 cycle this keeps
them in.

test/units/TEST-74-AUX-UTILS.detect-virt.sh

@@ -5,3 +5,7 @@ set -o pipefail
 
 SYSTEMD_IN_CHROOT=1 systemd-detect-virt --chroot
 (! SYSTEMD_IN_CHROOT=0 systemd-detect-virt --chroot)
+
+if ! systemd-detect-virt -c; then
+    unshare --mount-proc --fork --user --pid systemd-detect-virt --container
+fi