bloated postgres data folder, clean up

Hey dear list,
after a fex years of experiments and crash,
I ended up with a grossly bloated postgres folder.
I had about 8 Go of useless files.
All is in a virtualbox, so I'm sure to be able to reproduce exactly, and
fried my postgres folder a couple of time before getting it right.

Julien (Rouhaud) helped me to find those useless files via SQL.
The idea is to list files in postgres directory with `pg_ls_dir`, then to
check that the dir name correspond to something useful (using
pg_relation_filenode).
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https://gist.github.com/Remi-C/926eaee04d61a7245eb8
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To be sure I export the found files list,
then use oid2name to check that no file is recognized.

files can then be deleted (using plpythonu in my case).

So far a vacuum full analyze raise no errors.

Warning : for this to work, the SQL query must be sent while connected to
the database to clean.

Hope this may be useful
Cheers,
Rémi-C