Replace weave/common/mflagext with common/mflagext.

Removes duplication between weave and common and reduces the amount of sources to compile.

common/mflagext/listvar.go

@@ -1,31 +0,0 @@
-package mflagext
-
-import (
-	"fmt"
-
-	"github.com/weaveworks/common/mflag"
-)
-
-type listOpts struct {
-	value      *[]string
-	hasBeenSet bool
-}
-
-func ListVar(p *[]string, names []string, value []string, usage string) {
-	*p = value
-	mflag.Var(&listOpts{p, false}, names, usage)
-}
-
-func (opts *listOpts) Set(value string) error {
-	if opts.hasBeenSet {
-		(*opts.value) = append((*opts.value), value)
-	} else {
-		(*opts.value) = []string{value}
-		opts.hasBeenSet = true
-	}
-	return nil
-}
-
-func (opts *listOpts) String() string {
-	return fmt.Sprintf("%v", []string(*opts.value))
-}