[book] Improve PostgreSQL configuration section * Do not advice for specific PostgreSQL version (point to meta-packages if available in Linux distros); * First mention packages from Linux distros, put the link to official postgres documentation at the end; Closes #2725302.

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import ConfigParser

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try:

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    undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)

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except ConfigParser.NoOptionError:

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    pass # this conf. file was probably already migrated

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else:

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    config.global_set_option('undo-enabled', bool(undo_actions))