Split off CBlockHeader from CBlock

Cleaner and removes the need for the application-specific flags in
serialize.h.

src/serialize.h

@@ -50,10 +50,6 @@ enum
     SER_NETWORK         = (1 << 0),
     SER_DISK            = (1 << 1),
     SER_GETHASH         = (1 << 2),
-
-    // modifiers
-    SER_SKIPSIG         = (1 << 16),
-    SER_BLOCKHEADERONLY = (1 << 17),
 };
 
 #define IMPLEMENT_SERIALIZE(statements)    \