Parameterize the interface name in reactivatetest. [skip appveyor]

> uname -sr
Haiku 1
> ./testprogs/reactivatetest
reactivatetest: Neither lo0 nor lo could be opened: lo: No such device
  exists (SIOCGIFSTATS: Invalid Argument)

The loopback interface is "lo" in Linux and "lo0" in BSD systems, but in
Haiku it is "loop", so instead of hard-coding or guessing let's just
make it a mandatory command-line argument.  Also the user may want to
run the test on a non-loopback interface.

CHANGES

@@ -53,6 +53,7 @@ DayOfTheWeek, Month DD, YYYY / The Tcpdump Group
       Make Sun C build warning-free.
       Make FreeBSD, Linux and macOS builds warning-free.
       Print MAC addresses in findalldevstest.
+      Parameterize the interface name in reactivatetest.
       When necessary, trust the OS to implement ffs().
       Fix HCI_CHANNEL_MONITOR detection with musl libc.
       At build time require a proof of suitable snprintf(3) implementation in

testprogs/reactivatetest.c

@@ -39,20 +39,22 @@ The Regents of the University of California.  All rights reserved.\n";
 static void PCAP_NORETURN error(PCAP_FORMAT_STRING(const char *), ...) PCAP_PRINTFLIKE(1,2);
 
 int
-main(void)
+main(int argc, char **argv)
 {
 	char ebuf[PCAP_ERRBUF_SIZE];
 	pcap_t *pd;
 	int status = 0;
 
-	pd = pcap_open_live("lo0", 65535, 0, 1000, ebuf);
+	if (argc != 2)
+		error("expecting exactly one command-line argument with the interface name");
+	char *ifname = argv[1];
+
+	pd = pcap_open_live(ifname, 65535, 0, 1000, ebuf);
 	if (pd == NULL) {
-		pd = pcap_open_live("lo", 65535, 0, 1000, ebuf);
-		if (pd == NULL) {
-			error("Neither lo0 nor lo could be opened: %s",
-			    ebuf);
-		}
+		error("pcap_open_live() failed: %s",
+		    ebuf);
 	}
+
 	status = pcap_activate(pd);
 	if (status != PCAP_ERROR_ACTIVATED) {
 		if (status == 0)