fileutils: Use ModeType as the mask for mode checks

POSIX provides S_IS*(m) macros to portably interpret the mode type,
but does not define values for each type [1].  Alban pointed out that
st_mode is not a bitfield on Linux [2].  For example, Linux defines
[3]:

  S_IFBLK 060000
  S_IFDIR 040000
  S_IFCHR 020000

So 'm&S_IFCHR == S_IFCHR', for example, would succeed for both
character and block devices.  Go translates the system values to a
platform-agnostic bitfield [4], so the previous approach works on Go.
But it may be confusing for people used to the native non-bitfield
mode, so this commit moves us to an approach that does not rely on
Go's using a bitfield.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_stat.h.html
[2]: opencontainers/runtime-tools#308 (comment)
[3]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/stat.h?h=v4.16#n9
[4]: https://github.com/golang/go/blob/b0d437f866eb8987cde7e6550cacd77876f36d4b/src/os/types.go#L45

Author: wking

fileutils.go

@@ -24,7 +24,7 @@ func CopyFile(source string, dest string) error {
 	gid := int(st.Gid)
 
 	// Handle symlinks
-	if si.Mode()&os.ModeSymlink != 0 {
+	if si.Mode()&os.ModeType == os.Symlink {
 		target, err := os.Readlink(source)
 		if err != nil {
 			return err
@@ -76,7 +76,7 @@ func CopyFile(source string, dest string) error {
 	}
 
 	// Chmod the file
-	if !(si.Mode()&os.ModeSymlink == os.ModeSymlink) {
+	if !(si.Mode()&os.ModeType == os.ModeSymlink) {
 		if err := os.Chmod(dest, si.Mode()); err != nil {
 			return err
 		}