Add os_page_num to pg_buffercache

Hi hackers,

I was doing some more tests around ba2a3c2302f (pg_buffercache_numa) and
thought that seeing how buffers are spread across multiple OS pages (if that's
the case) thanks to the os_page_num field is good information to have.

The thing that I think is annoying is that to get this information (os_page_num):

- One needs to use pg_buffercache_numa (which is more costly/slower) than pg_buffercache
- One needs a system with NUMA support enabled

So why not also add this information (os_page_num) in pg_buffercache?

- It would make this information available on all systems, not just NUMA-enabled ones
- It would help understand the memory layout implications of configuration changes
such as database block size, OS page size (huge pages for example) and see how the
buffers are spread across OS pages (if that's the case).

So, please find attached a patch to $SUBJECT then.

Remarks:

- Maybe we could create a helper function to reduce the code duplication between
pg_buffercache_pages() and pg_buffercache_numa_pages()
- I think it would have made sense to also add this information while working
on ba2a3c2302f but (unfortunately) I doubt that this patch is candidate for v18
post freeze (it looks more a feature enhancement than anything else)
- It's currently doing the changes in pg_buffercache v1.6 but will need to
create v1.7 for 19 (if the above stands true)

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com