BUG #5734: autovacuum_enabled input should be validated, standardized.

The following bug has been logged online:

Bug reference:      5734
Logged by:          Mark Stosberg
Email address:      mark@summersault.com
PostgreSQL version: 9.0.1
Operating system:   FreeBSD
Description:        autovacuum_enabled input should be validated,
standardized.
Details: 

The "autovacuum_enabled" storage parameter claims to be a boolean type:
http://www.postgresql.org/docs/9.0/static/sql-createtable.html#SQL-CREATETAB
LE-STORAGE-PARAMETERS
... but it fails to behave a normal boolean.

Normally, you could set a boolean with a value of false, 'off' or 'f',
but you would always get back a value of 'f'  With this value, there is
no translation.

I think this kind of boolean should be handled like a standard
PostgreSQL boolean.

I noticed because the Slony code base has a hardcoded check for
"autovacuum_enabled=off", when a false value could also be stored as
autovacuum_enabled=f

We should be able to rely on this value being always returned as
"autovacuum_enabled='f'" just a normal boolean would.

    Mark