Allow Multiple SMB Dialects

[ahm3dgg]

Sticking to SMB 2.1.0 causes SMB Negotiate to fail on old windows server versions, the smb2 package allows for more if not specified

type Negotiator struct {
        // ....
	SpecifiedDialect      uint16   // if it's zero, clientDialects is used. (See feature.go for more details)
}

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