Some versions of the MSVC runtime library have a non-C99-compliant
vsnprintf(), which we want to avoid. On Windows, use snprintf() and
vsnprintf() for VS 2015 and later, where they both exist in
C99-compliant forms, and wrap _{v}snprintf_s() otherwise (they're
guaranteed to do the null termination that we want).
This can prevent bizarre failures if, for example, you've done a
configuration in the top-level source directory, leaving behind one
config.h file, and then do an out-of-tree build in another directory,
with different configuration options. This way, we always pick up the
same config.h, in the build directory.
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.
