cherry-pick 41c6b... - add client startup time

cherry-pick 41c6b8abc60f90e73ae4ad89b0a922638a61bbea (add client startup time)
w/ conflicts

add client startup time as an entry to debug.log (note: logged time in debug.log differs by a few seconds from the one displayed in the Debug window) / make ClientModel::formatClientStartupTime() return a QString
Author:    Philip Kaufmann <phil.kaufmann@t-online.de>

Conflicts:
	src/init.cpp
	src/qt/clientmodel.cpp
	src/qt/clientmodel.h
	src/qt/rpcconsole.cpp