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.

diff --cc src/basic/namespace-util.c
Simple merge