snmp: let handlers accept OID from a lesser prefix

Most table handlers do not expect to be given an OID whose prefix is
outside what they can handle. This is not a problem with the SMUX
implementation since it always correct the OID such that the prefix
matches. However, this is not the case for the AgentX
implementation. A new function, smux_header_table() is used to do this
normalization.
diff --git a/bgpd/bgp_snmp.c b/bgpd/bgp_snmp.c
index 36fd4ef..c8f2aa5 100644
--- a/bgpd/bgp_snmp.c
+++ b/bgpd/bgp_snmp.c
@@ -455,7 +455,9 @@
   static struct in_addr addr;
   struct peer *peer;
 
-  *write_method = NULL;
+  if (smux_header_table(v, name, length, exact, var_len, write_method)
+      == MATCH_FAILED)
+    return NULL;
   memset (&addr, 0, sizeof (struct in_addr));
 
   peer = bgpPeerTable_lookup (v, name, length, &addr, exact);
@@ -765,6 +767,9 @@
   if (! bgp)
     return NULL;
 
+  if (smux_header_table(v, name, length, exact, var_len, write_method)
+      == MATCH_FAILED)
+    return NULL;
   memset (&addr, 0, sizeof (struct prefix_ipv4));
 
   binfo = bgp4PathAttrLookup (v, name, length, bgp, &addr, exact);