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/lib/smux.h b/lib/smux.h
index b29fdc7..72b4eaf 100644
--- a/lib/smux.h
+++ b/lib/smux.h
@@ -75,6 +75,8 @@
                               size_t, int, oid [], size_t);
 extern int smux_header_generic (struct variable *, oid [], size_t *, 
                                 int, size_t *, WriteMethod **);
+extern int smux_header_table (struct variable *, oid *, size_t *, 
+			      int, size_t *, WriteMethod **);
 
 /* For traps, three OID are provided: