Format ClassB_send, get and apply dust limit based on outputs

**1. Format ClassB_send (14ae3228076e411f30259b06dff84c9cdbfeb81a)**

Mostly adding intendentions, renaming a few variables and moving parts around.
Print reason, if CreateTransaction fails.
`if (!wallet) return 5;` was removed altogether.

**2. Add function to determine dust limit (74a05029f36b9ba432f0f75cfe15ea3fce0bc12c)**

Get minimum output amount to be spent by an output based on scriptPubKey size in relation to the minimum relay fee.

The minimum relay fee can be defined as command-line argument `-minrelayfee=<amount>`.

As per default `-minrelayfee=1000` is used which results in the following dust limits:

- Pay-to-pubkey-hash (34 byte): 0.00000546 BTC
- Multisig, two compressed public keys (80 byte): 0.00000684 BTC
- Multisig, one compressed, one uncompressed public key (112 byte): 0.00000780 BTC
- Multisig, three compressed public keys (114 byte): 0.00000786 BTC
- Multisig, one uncompressed, two compressed public keys (146 byte): 0.00000882 BTC

Related:

- core.h: bool IsDust(int64_t nMinRelayTxFee)
- init.cpp: int64_t CTransaction::nMinRelayTxFee

**3. Replace MP_DUST_LIMIT by GetDustLimit(const CScript&) (de00a2c5e4ec2eb07774a77e85ab64321de69a87)**