multi: add optional version arg to decodescript rpc.

[dnldd]

This add an optional version parameter to the decodescript rpc. It defaults to version zero if not set.

[davecgh]

This is closer, but it still needs to add an additional test after the name: "decodescript", test for the optional parameter named decodescript optional to be consistent with the other tests. That will also expose that a new Uint16 function is needed to be consistent with the existing Uint, Uint32, and Uint64 functions now that one of the concrete command constructor parameters requires it.

[davecgh]

I approved this as it's sufficient until further script version enhancements are made although, as can be shown from the additional test output below, unsupported versions aren't really being handled as they probably should be, but they are at least marked as nonstandard.

$ dcrctl decodescript 76a914000000000000000000000000000000000000000088ac
{
  "asm": "OP_DUP OP_HASH160 0000000000000000000000000000000000000000 OP_EQUALVERIFY OP_CHECKSIG",
  "reqSigs": 1,
  "type": "pubkeyhash",
  "addresses": [
    "DsQxuVRvS4eaJ42dhQEsCXauMWjvopWgrVg"
  ],
  "p2sh": "DcvE76ks4SjLUymP6GLUZL4vSDwTJGiUS5T"
}

$ dcrctl decodescript 76a914000000000000000000000000000000000000000088ac 0
{
  "asm": "OP_DUP OP_HASH160 0000000000000000000000000000000000000000 OP_EQUALVERIFY OP_CHECKSIG",
  "reqSigs": 1,
  "type": "pubkeyhash",
  "addresses": [
    "DsQxuVRvS4eaJ42dhQEsCXauMWjvopWgrVg"
  ],
  "p2sh": "DcvE76ks4SjLUymP6GLUZL4vSDwTJGiUS5T"
}

$ dcrctl decodescript 76a914000000000000000000000000000000000000000088ac 1
{
  "asm": "OP_DUP OP_HASH160 0000000000000000000000000000000000000000 OP_EQUALVERIFY OP_CHECKSIG",
  "type": "nonstandard",
  "p2sh": "DcvE76ks4SjLUymP6GLUZL4vSDwTJGiUS5T"
}