config: move NetMode into its own micropackage

It's going to be used a bit more and pulling whole config just for one type is
a bit wrong.

pkg/config/netmode/netmode.go

@@ -0,0 +1,31 @@
+package netmode
+
+const (
+	// MainNet contains magic code used in the NEO main official network.
+	MainNet Magic = 0x004f454e // 5195086
+	// TestNet contains magic code used in the NEO testing network.
+	TestNet Magic = 0x744f454e // 1951352142
+	// PrivNet contains magic code usually used for NEO private networks.
+	PrivNet Magic = 56753 // docker privnet
+	// UnitTestNet is a stub magic code used for testing purposes.
+	UnitTestNet Magic = 0
+)
+
+// Magic describes the network the blockchain will operate on.
+type Magic uint32
+
+// String implements the stringer interface.
+func (n Magic) String() string {
+	switch n {
+	case PrivNet:
+		return "privnet"
+	case TestNet:
+		return "testnet"
+	case MainNet:
+		return "mainnet"
+	case UnitTestNet:
+		return "unit_testnet"
+	default:
+		return "net unknown"
+	}
+}