Upgrade BufFile to use int64 for byte positions

Upgrade BufFile to use int64 for byte positions

This change has the advantage of removing some weird type casts, caused
by offset calculations based on pgoff_t but saved as int (on older
branches we use off_t, which could be 4 or 8 bytes depending on the
environment).  These are safe currently because capped by
MAX_PHYSICAL_FILESIZE, but we would run into problems when to make
MAX_PHYSICAL_FILESIZE larger or allow callers of these routines to use a
larger physical max size on demand.

While on it, this improves BufFileDumpBuffer() so as we do not use an
offset for "availbytes".  It is not a file offset per-set, but a number
of available bytes.

This change should lead to no functional changes.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aUStrqoOCDRFAq1M@paquier.xyz