Includes: Cleanup around net main and wallet

-Move from .h to .cpp: in main, net and wallet
-Remove unnecessary #include "main.h"
-Cleanup some wallet files includes

src/qt/optionsmodel.cpp

@@ -13,7 +13,7 @@
 
 #include "amount.h"
 #include "init.h"
-#include "main.h"
+#include "main.h" // For DEFAULT_SCRIPTCHECK_THREADS
 #include "net.h"
 #include "txdb.h" // for -dbcache defaults