Bug #25518: Limit process count

apt-mirror forks an unlimited number of "apt-ftparchive" processes, which are
very IO intensive. This makes the system very unresponsive.
Same goes for "gzip", which is CPU intensive.

Limit both loops to the maximum number of threads.
Remove a very anoying double print, which breaks output formatting.

diff --git a/apt-mirror b/apt-mirror
index 7ac8b19..916035e 100755
--- a/apt-mirror
+++ b/apt-mirror
@@ -477,8 +477,6 @@ my $size_output = format_bytes($need_bytes);
 
 print "$size_output will be downloaded into archive.\n";
 
-print STDERR "$size_output will be downloaded into archive.\n";
-
 download_urls("archive", sort keys %urls_to_download);
 
 
@@ -509,6 +507,7 @@ sub collect_children {
 }
 
 if ( get_variable( "recreate_packages" ) eq 'yes' ) {
+	my $nthreads = get_variable("nthreads");
 	my @gzip_files = ();
 	my @children = ();
 	my $pid;
@@ -535,9 +534,13 @@ if ( get_variable( "recreate_packages" ) eq 'yes' ) {
 		if ( $pid == 0 ) {
 			chdir get_variable("mirror_path")."/$path";
 
-			exec "apt-ftparchive $filetype $source > $packages\n";
+			exec "exec apt-ftparchive $filetype $source > $packages\n";
 		}
 		push @children, $pid;
+		if ( scalar @children >= $nthreads ) {
+			my $dead = wait();
+			@childrens = grep { $_ != $dead } @childrens;
+		}
 	}
 	collect_children( @children );
 
@@ -552,9 +555,13 @@ if ( get_variable( "recreate_packages" ) eq 'yes' ) {
 		if ( $pid == 0 ) {
 			chdir get_variable("mirror_path")."/$path";
 
-			exec "gzip < $packages > $packages.gz\n";
+			exec "exec gzip < $packages > $packages.gz\n";
 		}
 		push @children, $pid;
+		if ( scalar @children >= $nthreads ) {
+			my $dead = wait();
+			@childrens = grep { $_ != $dead } @childrens;
+		}
 	}
 	collect_children( @children );
 } else {