core: use setreuid/setregid trick to create session keyring with right ownership...

core: use setreuid/setregid trick to create session keyring with right ownership (#8447)

Re-use the hacks used to link user keyring, when creating the session
keyring. This way changing ownership of the keyring is not required, and thus
incovation_id can be correctly created in restricted environments.

Creating invocation_id with root permissions works and linking it into session
keyring works, as at that point session keyring is possessed.

Simple way to validate this is with following commands:

$ journalctl -f &
$ sudo systemd-run --uid 1000 /bin/sh -c 'keyctl describe @s; keyctl list @s; keyctl read `keyctl search @s user invocation_id`'

which now works in LXD containers as well as on the host.

Fixes: https://github.com/systemd/systemd/issues/7655