rpcserver: only block co-op close for frozen chans
rpcserver: only block co-op close for frozen chans

This commit fixes a recent issue from #4081 that would prevent a frozen
channel from being force closed via the rpc. We correct this, so that
only the co-op path is inhibited.

LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd