Re: POC: make mxidoff 64 bits

One more small issue: The docs for pg_resetwal contain recipes for how 
to determine safe values to use:

> -m mxid,mxid
> --multixact-ids=mxid,mxid
> Manually set the next and oldest multitransaction ID.
> 
> A safe value for the next multitransaction ID (first part) can be
> determined by looking for the numerically largest file name in the
> directory pg_multixact/offsets under the data directory, adding one,
> and then multiplying by 65536 (0x10000). Conversely, a safe value
> for the oldest multitransaction ID (second part of -m) can be
> determined by looking for the numerically smallest file name in the
> same directory and multiplying by 65536. The file names are in
> hexadecimal, so the easiest way to do this is to specify the option
> value in hexadecimal and append four zeroes.
> 
> -O mxoff
> --multixact-offset=mxoff
> 
> Manually set the next multitransaction offset.
> 
> A safe value can be determined by looking for the numerically
> largest file name in the directory pg_multixact/members under the
> data directory, adding one, and then multiplying by 52352 (0xCC80).
> The file names are in hexadecimal. There is no simple recipe such as
> the ones for other options of appending zeroes.

I think those recipes need to be adjusted for 64-bit offsets.

- Heikki