fstype is broken on non-Linux operating systems

The package internal/helper/fstype has multiple problems on operating systems other than Linux:

• In detect_darwin.go, statFileSystemType erroneously tries to use Linux-specific magic values for syscall.Statfs_t.Type, which is always 0 on macOS.
• On other systems, e.g. FreeBSD or OpenBSD, statFileSystemType is undefined, thus breaking the build and the install process.