Add a --no-ftp mode to the release.pl download checks.

chiark's ftp server sometimes randomly refuses downloads. In the case
where this happens at postcheck time, this isn't really
release-blocking (all the files have been test-downloaded by precheck
already, so the main aim at this stage is to check that the 'latest'
symlink points to the right place, and even one or two successful
downloads are good enough to confirm that in practice). So now I can
add --no-ftp to the postcheck command line if that makes my life
easier.

release.pl

@@ -15,11 +15,13 @@ my $setver = 0;
 my $upload = 0;
 my $precheck = 0;
 my $postcheck = 0;
+my $skip_ftp = 0;
 GetOptions("version=s" => \$version,
            "setver" => \$setver,
            "upload" => \$upload,
            "precheck" => \$precheck,
-           "postcheck" => \$postcheck)
+           "postcheck" => \$postcheck,
+           "no-ftp" => \$skip_ftp)
     or &usage();
 
 # --set-version: construct a local commit which updates the version
@@ -163,11 +165,13 @@ if ($precheck || $postcheck) {
                    }
 
                    # Now test-download the files themselves.
-                   my $ftpdata = `curl -s $ftp_uri`;
-                   printf "  got %d bytes via FTP", length $ftpdata;
-                   die "FTP download for $ftp_uri did not match"
-                       if $ftpdata ne $real_content;
-                   print ", ok\n";
+                   unless ($skip_ftp) {
+                       my $ftpdata = `curl -s $ftp_uri`;
+                       printf "  got %d bytes via FTP", length $ftpdata;
+                       die "FTP download for $ftp_uri did not match"
+                           if $ftpdata ne $real_content;
+                       print ", ok\n";
+                   }
 
                    my $ua = LWP::UserAgent->new;
                    my $httpresponse = $ua->get($http_uri);