[PATCH] Teach pg_waldump to extract FPIs from the WAL

Hi -hackers,

Enclosed is a patch to allow extraction/saving of FPI from the WAL
stream via pg_waldump.

Description from the commit:

Extracts full-page images from the WAL stream into a target directory,
which must be empty or not
exist. These images are subject to the same filtering rules as normal
display in pg_waldump, which
means that you can isolate the full page writes to a target relation,
among other things.

Files are saved with the filename: <lsn>.<ts>.<db>.<rel>.<blk> with
formatting to make things
somewhat sortable; for instance:

00000000-010000C0.1663.1.6117.0
00000000-01000150.1664.0.6115.0
00000000-010001E0.1664.0.6114.0
00000000-01000270.1663.1.6116.0
00000000-01000300.1663.1.6113.0
00000000-01000390.1663.1.6112.0
00000000-01000420.1663.1.8903.0
00000000-010004B0.1663.1.8902.0
00000000-01000540.1663.1.6111.0
00000000-010005D0.1663.1.6110.0

It's noteworthy that the raw images do not have the current LSN stored
with them in the WAL
stream (as would be true for on-heap versions of the blocks), nor
would the checksum be valid in
them (though WAL itself has checksums, so there is some protection
there). This patch chooses to
place the LSN and calculate the proper checksum (if non-zero in the
source image) in the outputted
block. (This could perhaps be a targetted flag if we decide we don't
always want this.)

These images could be loaded/inspected via `pg_read_binary_file()` and
used in the `pageinspect`
suite of tools to perform detailed analysis on the pages in question,
based on historical
information, and may come in handy for forensics work.

Best,

David