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

implementation analogous to 298f35cdfbcfbf171fb7de645e90bdf6a368d6c8,
which is for addholdinvoice.

aakselrod/lnd,aakselrod/lnd