Fix currency/fee-rate unit string in the help text

Fix currency/fee-rate unit string in the help text

1. The RPC help text should use the constant CURRENCY_UNIT defined in
  policy/feerate.cpp instead of the literal 'BTC'. In the following
  2 RPC commands, 'BTC' is written directly in the help text.
    1) estimatesmartfee
    2) estimaterawfee
  And also, for these help strings, the notation
  'fee-per-kilobyte (in BTC)' is somewhat ambiguous.

  To write more precisely, this commit changes to 'fee rate in BTC/kB'
  with using the constant CURRENCY_UNIT.

2. Some RPC command use 'satoshis' as the unit. It should be written
  as 'satoshis' instead of 'Satoshis' in the RPC help text.

  So, this commit fixes this typo in getblocktemplate.

3. The phrase that '... feerate (BTC per KB) ...'  is used to explain
  the fee rate in the help text of following 2 RPC commands.
    1) getmempoolinfo
    2) fundrawtransaction
  But they are different from other similar help text of the RPCs.
  And also, 'KB' implies Kibibyte (2^10 byte).

  To unify and to clarify, this commit changes these phrase to
  '... fee rate in BTC/kB ...'.
  (BTC references the constant 'CURRENCY_UNIT')