The documentation for `--statedir` says that if unspecified, it will be derived
from `--state` if possible. It was not. In addition, if `--state` was specified,
`--statedir` was not even defaulted to its usual default, so remained empty.
Now, if only `--state` is specified, `--statedir` is derived from it.
Updates #14025
Signed-off-by: Percy Wegmann <percy@tailscale.com>