Doesn't compile under go1.12beta1

The package doesn't compile under go1.12beta1

I've tracked this down to a non-backwards compatible change in the FreeBSD go syscalls for which I've opened an issue

https://github.com/golang/go/issues/29393

Depending on the resolution of that issue I'll close this, or implement a version dependent work-around

The issue points to a syscall incompatibility on FreeBSD in Go 1.12beta1, linked to golang/go#29393. Start by checking the Go issue's resolution and examining the syscall usage in the tree package, likely in files handling OS-specific operations. Determine if a version-dependent workaround is needed by testing compilation under different Go versions. Done looks like the package compiling successfully on FreeBSD with Go 1.12+.

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