logind: change user-runtime-dir to query runtime dir size from logind via the bus

logind: change user-runtime-dir to query runtime dir size from logind via the bus

I think this is a slightly cleaner approach than parsing the
configuration file at multiple places, as this way there's only a single
reload cycle for logind.conf, and that's systemd-logind.service's
runtime.

This means that logind and dbus become a requirement of
user-runtime-dir, but given that XDG_RUNTIME_DIR is not set anyway
without logind and dbus around this isn't really any limitation.

This also simplifies linking a bit as this means user-runtime-dir
doesn't have to link against any code of logind itself.