(sql-magic-go): Use comint-bol.

(sql-magic-go): Use comint-bol.
(sql-copy-column): Use comint-line-beginning-position.
(comint-line-beginning-position): Define a replacement for
comint-line-beginning-position if it is not fboundp.

(sql-mode-map): TAB is no longer defined in sql-mode-map;
it didn't have any effect anyway.

(sql-postgres): Jason Beegan's patch uses the parameters
--pset and pager=off instead of sending \\o|cat at the beginning
of the session.