Plugin Interface Refactor

This makes two main changes to the plugin system:

* Instead of assuming that each plugin will have exactly one type,
  inspect each plugin to see which interfaces it provides, and
  register it as a provider of each provided interface. This can
  allow a single .so file to provide multiple interfaces, which
  will likely be necessary for aggregating certain types of info.
* Rather than using dependency injection and having to propagate
  the plugin system all throughout Geth, have a default plugin
  loader so we need only import the module and make calls to it.
  If the plan were to integrate this into mainline Geth, I would
  say we use dependency injection and take the time to pass the
  plugin loader throughout the codebase, but as I expect this to
  be a fork that has to pull upstream changes, this approach
  should make merge conflicts much less common.

cmd/geth/main.go

@@ -20,6 +20,7 @@ package main
 import (
 	"fmt"
 	"os"
+	"path"
 	"sort"
 	"strconv"
 	"strings"
@@ -39,6 +40,7 @@ import (
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/metrics"
 	"github.com/ethereum/go-ethereum/node"
+	"github.com/ethereum/go-ethereum/plugins"
 	"gopkg.in/urfave/cli.v1"
 )
 
@@ -311,13 +313,18 @@ func prepare(ctx *cli.Context) {
 // It creates a default node based on the command line arguments and runs it in
 // blocking mode, waiting for it to be shut down.
 func geth(ctx *cli.Context) error {
-	if args := ctx.Args(); len(args) > 0 {
-		return fmt.Errorf("invalid command: %q", args[0])
+	if err := plugins.Initialize(path.Join(ctx.GlobalString(utils.DataDirFlag.Name), "plugins")); err != nil { return err }
+	if ok, err := plugins.RunSubcommand(ctx); ok { return err }
+	if !plugins.ParseFlags(ctx.Args()) {
+		if args := ctx.Args(); len(args) > 0 {
+			return fmt.Errorf("invalid command: %q", args[0])
+		}
 	}
 
 	prepare(ctx)
 	stack, backend := makeFullNode(ctx)
 	defer stack.Close()
+	stack.RegisterAPIs(plugins.GetAPIs(stack, backend))
 
 	startNode(ctx, stack, backend)
 	stack.Wait()