sql.el should add support PostgreSQL alias "postgresql" of "postgres"

[stardiviner]

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Emacs built-in sql.el mode "sql-mode" is used by many extra extensions. Like Org
Mode, ejc-sql (https://github.com/kostafey/ejc-sql) etc.

Here is a discussion
(https://lists.gnu.org/archive/html/emacs-orgmode/2020-01/msg00347.html) of add
alias "postgresql" for "postgres" which comes from `sql-product-alist'.

And in ejc-sql I got this issue again because `sql-set-product' invoked in
ejc-sql but has no match of "postgresql" in `sql-product-alist' caused error.

So I hope sql.el can support "postgresql" name alias in `sql-product-alist'.

- -- 
[ stardiviner ]
       I try to make every word tell the meaning what I want to express.

       Blog: https://stardiviner.github.io/
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- -- 
[ stardiviner ]
       I try to make every word tell the meaning what I want to express.

       Blog: https://stardiviner.github.io/
       IRC(freenode): stardiviner, Matrix: stardiviner
       GPG: F09F650D7D674819892591401B5DF1C95AE89AC3
      
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