Clarify minfree warning message in check_space(..)

- State that the units are kB.
- Be more complete/concise in terms of what is required (in this case
  `minfree` must be at least `X`kB)

MFC after:	7 weeks
Sponsored by:	Dell EMC Isilon

sbin/savecore/savecore.c

@@ -303,7 +303,8 @@ check_space(const char *savedir, off_t dumpsize, int bounds)
 	needed -= saved_dump_size(bounds);
 	if ((minfree > 0 ? spacefree : totfree) - needed < minfree) {
 		syslog(LOG_WARNING,
-	"no dump, not enough free space on device (%lld available, need %lld)",
+		    "no dump: not enough free space on device (%lldkB "
+		    "available; need at least %lldkB)",
 		    (long long)(minfree > 0 ? spacefree : totfree),
 		    (long long)needed);
 		return (0);