Add DragonFlyBSD support

[daftaupe]

Hello,

I did

• move the linux part of process_unix.go to a process_linux.go and process_dragonfly.go (Path and Refresh methods)
• add a dragonfly build flag on process_unix.go
• add a dragonfly build flag on process_nix_test.go and process_unix_test.go

Regards,