replace cli with engine method, simplify

ethcore/src/engines/basic_authority.rs

@@ -99,6 +99,8 @@ impl Engine for BasicAuthority {
 	/// This assumes that all uncles are valid uncles (i.e. of at least one generation before the current).
 	fn on_close_block(&self, _block: &mut ExecutedBlock) {}
 
+	fn seals_internally(&self) -> bool { true }
+
 	/// Attempt to seal the block internally.
 	///
 	/// This operation is synchronous and may (quite reasonably) not be available, in which `false` will

ethcore/src/engines/instant_seal.rs

@@ -58,6 +58,8 @@ impl Engine for InstantSeal {
 		Schedule::new_homestead()
 	}
 
+	fn seals_internally(&self) -> bool { true }
+
 	fn generate_seal(&self, _block: &ExecutedBlock, _accounts: Option<&AccountProvider>) -> Option<Vec<Bytes>> {
 		Some(Vec::new())
 	}

ethcore/src/engines/mod.rs

@@ -71,6 +71,8 @@ pub trait Engine : Sync + Send {
 	/// Block transformation functions, after the transactions.
 	fn on_close_block(&self, _block: &mut ExecutedBlock) {}
 
+	/// If true, generate_seal has to be implemented.
+	fn seals_internally(&self) -> bool { false }
 	/// Attempt to seal the block internally.
 	///
 	/// If `Some` is returned, then you get a valid seal.