add constant for shared (GUI/core) -par settings

- introduce DEFAULT_SCRIPTCHECK_THREADS in main.h
- only show values from -"MAX_HW_THREADS" up to 16 for -par, as it
  makes no sense to try to leave more "cores free" than the system
  supports anyway
- use the new constant in optionsdialog and remove defaults from
  .ui file

Conflicts:
	src/init.cpp
	src/main.h
'sendrawtransaction' improvements

- Make it report the reject code and reason
- Make it possible to re-send transactions that are already in the mempool
remove an assignment which is never used.
Add nHighTransactionFeeWarning as per #3969.

Conflicts:
	src/wallet.h
Fix `-printblocktree` output

PrintBlockTree output was broken starting from e010af70.
Everything appears on one line.

PrintWallet() added the newline after a block, but this functionality
was removed and no newline was added.

Seemingly, no one noticed. Add a newline after the block information
to fix this.
build: improve missing boost error reporting
doc: Add note about memory reqs for compilation
qt: Don't require db_cxx.h when wallet disabled

Fix #3978.
Organize RPCCommands table

Use sensible categories (overall control, P2P, blockchain/UTXO and
mining, wallet, wallet-enabled mining) and sort within each.

Also remove unnecessary #ifdef ENABLE_WALLET from `rpcnet.cpp`.

Functionality-neutral change.

Conflicts:
	src/rpcserver.cpp
Fix typo in `createmultisig` help

iCreateMultisig is not a thing.

Conflicts:
	src/rpcmisc.cpp
Wrap create_directory calls in try...catch blocks.

Ignores any exceptions thrown if directory exists, otherwise re-throws exception.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>

Conflicts:
	src/util.cpp
Use the new name Bitcoin Core Daemon instead of Bitcoin server

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From-Github-Pull: #3801

Conflicts:
	src/bitcoin-cli.cpp
	src/bitcoind.cpp
Add sighash tests from data file
Add code generating data/sighash.json test data
Add application category for OSX

In "View by Category" without this metadata the app gets sorted into "Other" at the bottom.
add checks for deserialization errors
unified and better log/error messages for CDBEnv/CDB
[Qt] small cleanup of coincontroldialog

- use a little more Qt-style
- check for NULL pointers first and return in updateView()
- small space and formating changes
added many rpc wallet tests

deleted the empty no throw test in rpc_wallet_tests line 65

fixed some comments

starting verify tests

finished verify message tests

changed some comments
[Qt] rescan progress