smux: isolate SMUX implementation from SNMP implementation

smux: isolate SMUX implementation from SNMP implementation

lib/snmp.c gets OID related helper functions that can be used with
another SNMP interface. smux.h is cleaned of SMUX specific bits to
only expose functions that may be used by an alternative
implementation. We also do not redefine functions already present in
NetSNMP. Just use the appropriate headers.