Have a #define to squelch -Wunused-result warnings.

Have a #define to squelch -Wunused-result warnings.

In one place, we really *do* legitimately ignore the return value of
pcap_set_datalink(). We cast its return value to void, which appears to
squelch warnings from Clang, but not from at least some versions of GCC,
so provide DIAG_OFF_WARN_UNUSED_RESULT/DIAG_ON_WARN_UNUSED_RESULT and
use them to squelch the warning.