Have the call to smiLoadModule() be in a nd_load_smi_module() routine.
Have it set a *global* flag indicating whether a module has been loaded;
that's not per-netdissect_options. Use that global flag in print-snmp.c
- and don't test it once per loop iteration, it's not going to change
while the loop is running.
Have a routine to return the version of the library if we're built with
it or NULL if we're not.
That removes the last of the code that tests USE_LIBSMI or uses libsmi
from tcpdump.c.