runc features: add seccomp filter flags

Amend runc features to print seccomp flags. Two set of flags are added:
 * known flags are those that this version of runc is aware of;
 * supported flags are those that can be set; normally, this is the same
   set as known flags, but due to older version of kernel and/or
   libseccomp, some known flags might be unsupported.

This commit also consolidates three different switch statements dealing
with flags into one, in func setFlag. A note is added to this function
telling what else to look for when adding new flags.

Unfortunately, it also adds a list of known flags, that should be
kept in sync with the switch statement.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

features.go

@@ -59,10 +59,12 @@ var featuresCommand = cli.Command{
 
 		if seccomp.Enabled {
 			feat.Linux.Seccomp = &features.Seccomp{
-				Enabled:   &tru,
-				Actions:   seccomp.KnownActions(),
-				Operators: seccomp.KnownOperators(),
-				Archs:     seccomp.KnownArchs(),
+				Enabled:        &tru,
+				Actions:        seccomp.KnownActions(),
+				Operators:      seccomp.KnownOperators(),
+				Archs:          seccomp.KnownArchs(),
+				KnownFlags:     seccomp.KnownFlags(),
+				SupportedFlags: seccomp.SupportedFlags(),
 			}
 			major, minor, patch := seccomp.Version()
 			feat.Annotations[features.AnnotationLibseccompVersion] = fmt.Sprintf("%d.%d.%d", major, minor, patch)