Fix future version compatibility

In case kernel folks will ever release capability v4, the chances are
high v3 is still supported. Therefore, we should not error out upon
seeing an unknown version from the kernel, but assume we can go with v3.

While at it, treat the uninitialized capVers as an error. Before this
patch, it was still treated as an error, but "unknown capability version"
is not exactly what the error is, so let's be more specific.

Reported-by: Andrei Vagin <avagin@google.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

capability_linux.go

@@ -16,8 +16,6 @@ import (
 	"syscall"
 )
 
-var errUnknownVers = errors.New("unknown capability version")
-
 const (
 	linuxCapVer1 = 0x19980330
 	linuxCapVer2 = 0x20071026
@@ -100,19 +98,21 @@ func mkString(c Capabilities, max CapType) (ret string) {
 
 func newPid(pid int) (c Capabilities, err error) {
 	switch capVers {
+	case 0:
+		err = errors.New("unable to get capability version from the kernel")
 	case linuxCapVer1:
 		p := new(capsV1)
 		p.hdr.version = capVers
 		p.hdr.pid = int32(pid)
 		c = p
-	case linuxCapVer2, linuxCapVer3:
+	default:
+		// linuxCapVer2, linuxCapVer3, or an unknown/future version such as v4.
+		// In the latter case, we fall back to v3 hoping the kernel is
+		// backward-compatible to v3.
 		p := new(capsV3)
 		p.hdr.version = capVers
 		p.hdr.pid = int32(pid)
 		c = p
-	default:
-		err = errUnknownVers
-		return
 	}
 	return
 }