core: add new per-unit setting KeyringMode= for controlling kernel keyring setup

core: add new per-unit setting KeyringMode= for controlling kernel keyring setup

Usually, it's a good thing that we isolate the kernel session keyring
for the various services and disconnect them from the user keyring.
However, in case of the cryptsetup key caching we actually want that
multiple instances of the cryptsetup service can share the keys in the
root user's user keyring, hence we need to be able to disable this logic
for them.

This adds KeyringMode=inherit|private|shared:

    inherit: don't do any keyring magic (this is the default in systemd --user)
    private: a private keyring as before (default in systemd --system)
    shared: the new setting