Patch to remove sort files, temp tables, unreferenced files

The following patch does full referential checking of sort files, temp
tables, and table files. It checks references of every file and temp
table, and reports or removes it. The code only removes files it is
certain about.

A typical output report is:

test=> VACUUM;
NOTICE: Unusual file found in temporary sort directory. This file is not
normally created by PostgreSQL and can be removed by the
administrator using 'rm':
/usr/var/local/postgres/data/base/18617/pg_sorttemp/x
NOTICE: Unusual file found in database directory. This file is not
normally created by PostgreSQL and can be removed by the
administrator using 'rm':
/usr/var/local/postgres/data/base/18617/x
NOTICE: Core file found in database directory. If you don't need it
for debugging, the administrator can remove it using 'rm':
/usr/var/local/postgres/data/base/18617/core
NOTICE: Unreferenced table file found in database directory. This
could have been left from a database crash. If no one was
using the database during VACUUM, the file can be safely
removed by the administrator using 'rm':
/usr/var/local/postgres/data/base/18617/33333

Most of the work is done as part of a full database VACUUM. Postmaster
startup also cleans all sort files.

It uses a SnapshotAny read of pg_class to find referenced files. It
does not use Oid ranges anymore.

I also had to add ShmemPIDAdd() and redo ShmemPIDLookup() because the
ShmemPIDLookup() API was weird.

--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
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