add missing Boost Thread join_all() call during shutdown

- fixes #3037 by adding missing join_all() call and brings bitcoind
  shutdown code in line with Bitcoin-Qt shutdown code
- added a comment for the if (!fRet) case

Conflicts:
	src/bitcoind.cpp
build: cosmetic: split usage string for easier formatting

Conflicts:
	src/init.cpp
build: use runtime setting for displaying the help message rather than QT_GUI define

Conflicts:
	src/init.cpp
build: use runtime setting for wallet rather than QT_GUI define
build: cosmetics after last commit
build: kill off the QT_GUI define

Conflicts:
	opalcoin-qt.pro