transport_darwin is the right way for all (non-linux) unix systems

[reezer]

In #33 it was decided to not pin the Ucred calls to Linux only. However this would have been the right thing to do, even for BSDs.

See these for reference:

• https://github.com/golang/sys/blob/master/unix/sockcmsg_linux.go
• https://github.com/golang/sys/blob/master/unix/sockcmsg_unix.go