netsniff-ng: Use macros for printf/scanf format specifiers

Any types that are fixed width should use the standard format specifier
macros (PRI... for printf-type functions, SCN... for scanf-type
functions) to ensure proper data access.

Prior to this ifpps was crashing in 32-bit environments due to the
following call

```C
	mvwprintw(screen, (*voff)++, 2,
		  "%s,%s %s (%s%s), t=%lums, cpus=%u%s/%u"
		  "               ", uts.release, machine,
		  ifname, drvinf.driver, buff, ms_interval, top_cpus,
		  top_cpus > 0 && top_cpus < cpus ? "+1" : "", cpus);
```

since ms_interval is a uint64_t but %lu expects an unsigned long, which
is only 32 bits.

Signed-off-by: James McCoy <vega.james@gmail.com>