cmd/util: rename OS specific files

Update file names based on their GOOS, allowing the removal of the
explicit build constraint, `// +build`, calls. Additionally, this patch
rename the _darwin specific file to _bsd, to allow future addition of BSD
systems.

Signed-off-by: Bruno Meneguele <bmeneg@redhat.com>

cmd/util_darwin.go → cmd/util_bsd.go

@@ -1,4 +1,4 @@
-// This file contains Darwin (MacOS) specific calls.
+// This file contains Darwin (MacOS) and *BSD specific calls.
 
 // +build darwin
 
@@ -8,8 +8,8 @@ package cmd
 // by Linux ARM64 not having the DUP2() anymore. With that, we need to
 // repeat the other code and func declarations that are the same.
 
-// FIXME: there MUST be some better way to do that... only dupFD2() should be
-// here.
+// FIXME: there MUST be some better way to do that... only dupFD2() should
+// be here.
 
 import "syscall"
 

cmd/util_unix.go → cmd/util_linux.go

@@ -1,13 +1,10 @@
 // This file contains Linux specific calls.
 
-// +build !windows,!darwin
-
 package cmd
 
 // Since we're using some system calls that are platform-specific, we need
-// to make sure we have a small layer of compatibility for Unix-like and
-// Windows operating systems. For now, this file is still valid for BSDs
-// (MacOS NOT included)
+// to make sure we have a small layer of compatibility for Linux, Windows
+// and *BSD operating systems.
 
 import "syscall"
 

cmd/util_windows.go

@@ -1,7 +1,5 @@
 // This file contains Windows specific calls.
 
-// +build windows
-
 package cmd
 
 // Even though Windows has a POSIX layer, it's implemented in userspace and,