session: add offset to SQL session settings array

Session settings are stored in a monolithic array. Submodules
can define a range of settings in there. For example, SQL. It
occupies settings from 0 to 8. There is a second array of only
SQL settings in build.c, of the same size, and it uses the same
indexes.

But if something will be added before SQL settings, so as they
won't start from 0, it will break the equal indexes assumption.
SQL should normalize all setting identifiers by
SESSION_SETTING_SQL_BEGIN.