Babel: Parse sub-TLVs and skip TLVs with mandatory sub-TLV

RFC6126bis formally introduces sub-TLVs to the Babel protocol, including
mandatory sub-TLVs. This adds support for parsing sub-TLVs to the Babel
protocol and skips TLVs that contain mandatory sub-TLVs, as per the spec.

For details, see section 4.4 of
https://tools.ietf.org/html/draft-ietf-babel-rfc6126bis-02

Thanks to Toke Høiland-Jørgensen <toke@toke.dk> for the patch.

proto/babel/babel.h

@@ -78,6 +78,11 @@ enum babel_tlv_type {
   BABEL_TLV_MAX
 };
 
+enum babel_subtlv_type {
+  BABEL_SUBTLV_PAD1		= 0,
+  BABEL_SUBTLV_PADN		= 1
+};
+
 enum babel_iface_type {
   /* In practice, UNDEF and WIRED give equivalent behaviour */
   BABEL_IFACE_TYPE_UNDEF	= 0,