Support decoding of catalog tables.

Support decoding of catalog tables.

Support decoding of 'name' and 'char' types. Also introduce '~'
pseudo-type which just ignores all data left in current tuple during
decoding. These two types allow easily to decode catalog tables and
restore schema of a database even if it's corrupted and PostgreSQL
instance is not starting.

For instance, pg_attribute can be decoded like this:

pg_filedump -D oid,name,oid,int,smallint,~ path/to/segment/1249

Aleksander Alekseev