cmd/utils, node: create account manager in package node

The account manager was previously created by packge cmd/utils as part
of flag processing and then passed down into eth.Ethereum through its
config struct. Since we are starting to create nodes which do not have
eth.Ethereum as a registered service, the code was rearranged to
register the account manager as its own service. Making it a service is
ugly though and it doesn't really fix the root cause: creating nodes
without eth.Ethereum requires duplicating lots of code.

This commit splits utils.MakeSystemNode into three functions, making
creation of other node/service configurations easier. It also moves the
account manager into Node so it can be used by those configurations
without requiring package eth.

node/service.go

@@ -20,6 +20,7 @@ import (
 	"path/filepath"
 	"reflect"
 
+	"github.com/ethereum/go-ethereum/accounts"
 	"github.com/ethereum/go-ethereum/ethdb"
 	"github.com/ethereum/go-ethereum/event"
 	"github.com/ethereum/go-ethereum/p2p"
@@ -30,9 +31,10 @@ import (
 // the protocol stack, that is passed to all constructors to be optionally used;
 // as well as utility methods to operate on the service environment.
 type ServiceContext struct {
-	datadir  string                   // Data directory for protocol persistence
-	services map[reflect.Type]Service // Index of the already constructed services
-	EventMux *event.TypeMux           // Event multiplexer used for decoupled notifications
+	datadir        string                   // Data directory for protocol persistence
+	services       map[reflect.Type]Service // Index of the already constructed services
+	EventMux       *event.TypeMux           // Event multiplexer used for decoupled notifications
+	AccountManager *accounts.Manager        // Account manager created by the node.
 }
 
 // OpenDatabase opens an existing database with the given name (or creates one