lncli addholdinvoice: allow specifying msat with --amt_msat
lncli addholdinvoice: allow specifying msat with --amt_msat

* pass amt and amt_msat to rpc, letting server give an error if both
are present
* take amt from an extra argument if neither amt nor amt_ms are present

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