psql: Rename meta-command \close to \close_prepared

psql: Rename meta-command \close to \close_prepared

\close has been introduced in d55322b0da60 to be able to close a
prepared statement using the extended protocol in psql.  Per discussion,
the name "close" is ambiguous.  At the SQL level, CLOSE is used to close
a cursor.  At protocol level, the close message can be used to either
close a statement or a portal.

This patch renames \close to \close_prepared to avoid any ambiguity and
make it clear that this is used to close a prepared statement.  This new
name has been chosen based on the feedback from the author and the
reviewers.

Author: Anthonin Bonnefoy <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Reviewed-by: Jelte Fennema-Nio <[email protected]>
Discussion: https://postgr.es/m/3e694442-0df5-4f92-a08f-c5d4c4346b85@eisentraut.org