Update boot_unix.go to be compatible with terminal

It seems ReadPassword() assumes a different version of kless/terminal. In master:HEAD version of kless/terminal, terminal.ReadPassword() only accepts one parameter, as a result, shout cannot be built(Linux 64bit, go1). This change makes shout consistent with kless/terminal.
Merge pull request #1 from songgao/patch-1

Update boot_unix.go to be compatible with terminal