managen: output tabs for each 8 leading spaces

This replacing of eight leading spaces into tabs was already done for
the embedded uncompressed version in tool_hugehelp.c so it does not save
anything there. But the gzip compressed version ends up almost 2K
smaller.

The output in a terminal should be identical.

Before using TABs:

curl.txt 282492 bytes
curl.txt.gz 73261 bytes

With this change applied:

curl.txt 249382 bytes
curl.txt.gz 71470 bytes

Closes #14016

src/mkhelp.pl

@@ -167,6 +167,7 @@ for my $n (@out) {
     chomp $n;
     $n =~ s/\\/\\\\/g;
     $n =~ s/\"/\\\"/g;
+    $n =~ s/\t/\\t/g;
 
     if(!$n) {
         $blank++;