global: switch to x/sys/unix

global: switch to x/sys/unix

The syscall package is locked since 1.4[1], all new changes are in x/sys[2]
The fixes for epoll/arm64 are going to be loaded to x/sys/unix. This
commit is straightforward search/replace with a couple of hand edits.

This is needed for fixing #130

[1] https://golang.org/s/go1.4-syscall
[2] https://godoc.org/golang.org/x/sys/unix

closes #135