settings page incorrectly storing things in both the {variable} and {signup} tables

in testing some other stuff, i just noticed that we're needlessly storing all our settings in the {variable} table, because of how we're using system_settings_form(). given the non-standard way we're handling the default signup settings in the {signup} table, we should consider if we want to use system_settings_form() at all, and perhaps just add those buttons manually, and just have our own submit handler (which we already have and need) do whatever manipulations of the {variable} table are required...