Allow forcing nullness of columns during bootstrap.

Andres Freund <andres@anarazel.de>

Commit: eb68379c38202180bc8e33fb9987284e314b7fc8
Author: Andres Freund <andres@anarazel.de>
Date: 2015-02-21T21:31:54Z
Releases: 9.5.0
Allow forcing nullness of columns during bootstrap.

Bootstrap determines whether a column is null based on simple builtin
rules. Those work surprisingly well, but nonetheless a few existing
columns aren't set correctly. Additionally there is at least one patch
sent to hackers where forcing the nullness of a column would be helpful.

The boostrap format has gained FORCE [NOT] NULL for this, which will be
emitted by genbki.pl when BKI_FORCE_(NOT_)?NULL is specified for a
column in a catalog header.

This patch doesn't change the marking of any existing columns.

Discussion: 20150215170014.GE15326@awork2.anarazel.de

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