core,journal: export user units' InvocationID and use as _SYSTEMD_INVOCATION_ID

Write a user unit's invocation ID to /run/user/<uid>/systemd/units/ similar
to how a system unit's invocation ID is written to /run/systemd/units/.

This lets the journal read and add a user unit's invocation ID to the
_SYSTEMD_INVOCATION_ID field of logs instead of the user manager's
invocation ID.

Fixes #12474