Clean up configure check for libsmi.

First, check for smi.h.  If we don't have it, don't check for anything
else.

If we do have it, check for libsmi containing smiInit.  If we don't have
it, don't check for anything else.

If we do have it, check, with our test program, whether we can use it.

If that succeeds, prepend -lsmi to LIBS, and set USE_LIBSMI.  Otherwise,
don't do either of those.

Check, in source, *only* for USE_LIBSMI.  If it's set, use libsmi,
otherwise don't - don't even include smi.h, even if we happened to have
found it, and don't print the libsmi version string.
Document --with-sandbox-capsicum.