internal/flags: fix "flag redefined" bug for alias on custom flags

[gzliudan]

If we define an alias for any of DirectoryString and BigFlag, geth or make test will panic with message flag redefined. For example:

	AncientFlag = &flags.DirectoryFlag{
		Name:     "datadir.ancient",
		Aliases:  []string{"datadir-ancient"},
		Usage:    "Root directory for ancient data (default = inside chaindata)",
		Category: flags.EthCategory,
	}

The geth will panic with the following message

geth flag redefined: datadir.ancient
panic: geth flag redefined: datadir.ancient

Some of test cases will fail with the following message when we run make test:

    test_cmd.go:261: (stderr:1) geth-test flag redefined: datadir.ancient
    test_cmd.go:261: (stderr:1) panic: geth-test flag redefined: datadir.ancient

[holiman]

Nice catch, thanks!