core: evaluate presets after generators have run (#6526)

core: evaluate presets after generators have run (#6526)

This commit moves the first-boot system preset-settings evaluation out
of main and into the manager startup logic itself. Notably, it reverses
the order between generators and presets evaluation, so that any changes
performed by first-boot generators are taken into the account by presets
logic.

After this change, units created by a generator can be enabled as part
of a preset.