Remove security vulnerabilities that allowed connected users to read backend memory (Tom)

The vulnerabilities involve suppressing the normal check that a SQL function returns the data type it's declared to, and changing the data type of a table column (CVE-2007-0555, CVE-2007-0556). These errors can easily be exploited to cause a backend crash, and in principle might be used to read database content that the user should not be able to access.

Fix rare bug wherein btree index page splits could fail due to choosing an infeasible split point (Heikki Linnakangas)

Improve VACUUM performance for databases with many tables (Tom)

Fix autovacuum to avoid leaving non-permanent transaction IDs in non-connectable databases (Alvaro)

This bug affects the 8.1 branch only.

Fix for rare Assert() crash triggered by UNION (Tom)

Tighten security of multi-byte character processing for UTF8 sequences over three bytes long (Tom)

Fix bogus “permission denied” failures occurring on Windows due to attempts to fsync already-deleted files (Magnus, Tom)

Fix possible crashes when an already-in-use PL/pgSQL function is updated (Tom)