Replace off_t by pgoff_t in I/O routinesHEADmaster

PostgreSQL's Windows port has never been able to handle files larger
than 2GB due to the use of off_t for file offsets, only 32-bit on
Windows.  This causes signed integer overflow at exactly 2^31 bytes when
trying to handle files larger than 2GB, for the routines touched by this
commit.

Note that large files are forbidden by ./configure (3c6248a828af) and
meson (recent change, see 79cd66f28c65).  This restriction also exists
in v16 and older versions for the now-dead MSVC scripts.

The code base already defines pgoff_t as __int64 (64-bit) on Windows for
this purpose, and some function declarations in headers use it, but many
internals still rely on off_t.  This commit switches more routines to
use pgoff_t, offering more portability, for areas mainly related to file
extensions and storage.

These are not critical for WAL segments yet, which have currently a
maximum size allowed of 1GB (well, this opens the door at allowing a
larger size for them).  This matters more for segment files if we want
to lift the large file restriction in ./configure and meson in the
future, which would make sense to remove once/if all traces of off_t are
gone from the tree.  This can additionally matter for out-of-core code
that may want files larger than 2GB in places where off_t is four bytes
in size.

Note that off_t is still used in other parts of the tree like
buffile.c, WAL sender/receiver, base backup, pg_combinebackup, etc.
These other code paths can be addressed separately, and their update
will be required if we want to remove the large file restriction in the
future.  This commit is a good first cut in itself towards more
portability, hopefully.

On Unix-like systems, pgoff_t is defined as off_t, so this change only
affects Windows behavior.

Author: Bryan Green <[email protected]>
Reviewed-by: Thomas Munro <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Discussion: https://postgr.es/m/[email protected]

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