Rationalize currency unit to "BTC"

Previously various user-facing strings have used inconsistent currency units "BTC",
"btc" and "bitcoins". This adds a single constant and uses it for each reference to
the currency unit.

Also adds a description of the unit for --maxtxfee, and adds the missing "amount"
field description to the (deprecated) move RPC command.

src/amount.h

@@ -16,6 +16,8 @@ typedef int64_t CAmount;
 static const CAmount COIN = 100000000;
 static const CAmount CENT = 1000000;
 
+extern const std::string CURRENCY_UNIT;
+
 /** No amount larger than this (in satoshi) is valid.
  *
  * Note that this constant is *not* the total money supply, which in Bitcoin