Re: Add -k/--link option to pg_combinebackup

Thanks for the review, Robert!
I applied all of your suggestions.

> I'm still not a fan of the changes to 010_links.pl; let's drop those.

As discussed through a side chat, 010_links.pl is the new test file
which ensures the hard links are created as expected in the output
directory.

I've removed the changes that the v2 patch had in the other test files,
which were the ones you were suggesting to drop here.

> cfbot is fairly unhappy with your patch. See
> http://cfbot.cputube.org/israel-barth.html or
> https://cirrus-ci.com/github/postgresql-cfbot/postgresql/cf%2F5508 --
> I haven't looked into what is going wrong here, and there may well be
> more than one thing, but nothing can be committed until this is all
> green.

There are some failures on compiler warnings and Windows jobs that
seem unrelated with this patch.

Besides that, there were failures when executing `010_links.pl` in the
Linux autoconf jobs. As discussed in a side chat with Euler and Robert,
and pointed out by Robert, Cirrus CI uses a very small
`--with-segsize-blocks`. My tests in the V2 patch were assuming 1GB
segments, so they were failing in Cirrus CI.

The patch V3, which I'm attaching in this reply, implements some logic
to handle any segment size.

Best regards,
Israel.