Make bitcoin compile without wallet if "db_cxx.h" is not present

Moved includes of "db.h" into #ifdef ENABLE_WALLET blocks or remove
them.
qt: Make sure overviewpage button is pressed at startup

Due to walletframe changes, the overview page button is no longer
automatically selected at startup even though the overview page is shown.
Merge pull request #3438

d2b6de0 qt: Make sure overviewpage button is pressed at startup (Wladimir J. van der Laan)