Fix buildtag for go1.7 introduced in #77
Fix buildtag for go1.7 introduced in #77

In #77, `!go1.7` was added to `//go:build` in syscall_dup.go
despite it having `// +build go1.7` tag.

This automatically fixes build on go1.17/FreeBSD, and maybe other
platforms too.

sevlyar/go-daemon