main: Rename run_dir to state_dir

system.run_dir was a little bit unfortunate and confusing name. Rename
to state_dir makes more evident what is content of this directory. To
keep setting consistent with code, get_run_dir is changed to
get_state_dir.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>