Make the "Invalid adapter index" look better with 0 or 1 interface.

If *no* interfaces were found, just say "no interfaces available for
capture", as we do if no -i flag was specified and there aren't any
interfaces.

Otherwise, say "interface" rathr than "interfaces" if there's only one.

Show that and another message in the CHANGES section for changes
backported to 4.99.

Author: guyharris

CHANGES

@@ -116,6 +116,10 @@ DayOfTheWeek, Month DD, YYYY / The Tcpdump Group
       Update the -z option processing. Print "invalid option -- z" when it is.
     Other:
       Avoid race condition when receiving signal during shutdown.
+      Always show the interface name in error messages when the
+        interface can't be opened.
+      Clean up the message shown if a -i flag is specified with an
+        interface index that's too large.
     Source code:
       Fix '-tt' option printing when time > 2106-02-07T06:28:15Z.
       Add sub-second packet timestamp checks for invalid micro/nano.

tcpdump.c

@@ -1198,6 +1198,8 @@ _U_
 	status = pcap_findalldevs(&devlist, ebuf);
 	if (status < 0)
 		error("%s", ebuf);
+	if (devlist == NULL)
+		error("no interfaces available for capture");
 	/*
 	 * Look for the devnum-th entry in the list of devices (1-based).
 	 */
@@ -1206,8 +1208,8 @@ _U_
 		;
 	if (dev == NULL) {
 		pcap_freealldevs(devlist);
-		error("Invalid adapter index %ld: only %ld interfaces found",
-		    devnum, i);
+		error("Invalid adapter index %ld: only %ld interface%s found",
+		    devnum, i, (i == 1) ? "" : "s");
 	}
 	device = strdup(dev->name);
 	pcap_freealldevs(devlist);