Fixing several issues with my previous commit:
Use increment [by], no step.
Enable again code in ini.epp because it's needed when restoring (but not when creating gens directly in a new db).
Gbak has code to fix system generators and we should allow it.
Nbackup cannot update the history generator when doing a backup.
Field name renamed to rdb$generator_increment.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59404 65644016-39b1-43b1-bf79-96bc8fe82c15
Documentation for the generator increment.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59405 65644016-39b1-43b1-bf79-96bc8fe82c15
Bad name "Buffer" and misc cleanup around

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59406 65644016-39b1-43b1-bf79-96bc8fe82c15
Restored original error code when database file not found

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59407 65644016-39b1-43b1-bf79-96bc8fe82c15