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.cpp

@@ -7,6 +7,8 @@
 
 #include "tinyformat.h"
 
+const std::string CURRENCY_UNIT = "BTC";
+
 CFeeRate::CFeeRate(const CAmount& nFeePaid, size_t nSize)
 {
     if (nSize > 0)
@@ -27,5 +29,5 @@ CAmount CFeeRate::GetFee(size_t nSize) const
 
 std::string CFeeRate::ToString() const
 {
-    return strprintf("%d.%08d BTC/kB", nSatoshisPerK / COIN, nSatoshisPerK % COIN);
+    return strprintf("%d.%08d %s/kB", nSatoshisPerK / COIN, nSatoshisPerK % COIN, CURRENCY_UNIT);
 }