treewide: Replace strerror() calls

Now that we have logging functions embedding perror() functionality,
we can make _some_ calls more terse by using them. In many places,
the strerror() calls are still more convenient because, for example,
they are used in flow debugging functions, or because the return code
variable of interest is not 'errno'.

While at it, convert a few error messages from a scant perror style
to proper failure descriptions.

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

pcap.c

@@ -89,10 +89,8 @@ static void pcap_frame(const struct iovec *iov, size_t iovcnt,
 	struct iovec hiov = { &h, sizeof(h) };
 
 	if (write_remainder(pcap_fd, &hiov, 1, 0) < 0 ||
-	    write_remainder(pcap_fd, iov, iovcnt, offset) < 0) {
-		debug("Cannot log packet, length %zu: %s",
-		      l2len, strerror(errno));
-	}
+	    write_remainder(pcap_fd, iov, iovcnt, offset) < 0)
+		debug_perror("Cannot log packet, length %zu", l2len);
 }
 
 /**
@@ -178,5 +176,5 @@ void pcap_init(struct ctx *c)
 	info("Saving packet capture to %s", c->pcap);
 
 	if (write(pcap_fd, &pcap_hdr, sizeof(pcap_hdr)) < 0)
-		warn("Cannot write PCAP header: %s", strerror(errno));
+		warn_perror("Cannot write PCAP header");
 }