add InitMessage() to noui and use debug.log for GUI

- this pull adds an InitMessage() function to noui.cpp, which outputs init
  messages to debug.log (this allows to remove some printf() calls from
  init.cpp)
- change InitMessage() in bitcoin.cpp to also write init messages to
  debug.log to ensure nothting is missing in the log because of the
  removal of printf() calls in init.cpp