arp, tap, util: Don't use perror() after seccomp filter is installed

If stderr is closed, after we fork to background, glibc's
implementation of perror() will try to re-open it by calling dup(),
upon which the seccomp filter causes the process to terminate,
because dup() is not included in the list of allowed syscalls.

Replace perror() calls that might happen after isolation_postfork().
We could probably replace all of them, but early ones need a bit more
attention as we have to check whether log.c functions work in early
stages.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

tap.c

@@ -899,7 +899,7 @@ static void tap_sock_unix_init(struct ctx *c)
 	int i;
 
 	if (fd < 0) {
-		perror("UNIX socket");
+		err("UNIX socket: %s", strerror(errno));
 		exit(EXIT_FAILURE);
 	}
 
@@ -920,7 +920,7 @@ static void tap_sock_unix_init(struct ctx *c)
 
 		ex = socket(AF_UNIX, SOCK_STREAM | SOCK_NONBLOCK, 0);
 		if (ex < 0) {
-			perror("UNIX domain socket check");
+			err("UNIX domain socket check: %s", strerror(errno));
 			exit(EXIT_FAILURE);
 		}
 
@@ -944,7 +944,7 @@ static void tap_sock_unix_init(struct ctx *c)
 	}
 
 	if (i == UNIX_SOCK_MAX) {
-		perror("UNIX socket bind");
+		err("UNIX socket bind: %s", strerror(errno));
 		exit(EXIT_FAILURE);
 	}