preflight: reliability and misc fixes

 - Replaced limited retries with silent but continuous retries
 - Eliminated selective refetching of params (always refetch)
 - Limit number of refetched params per timeout
 - Made timeout and refetch number class-level variables
 - Generalized PARAMS_MIN into PARAMS_BOUNDS with (min, max)
 - Check that *exactly* 1 rally point exists
 - Fixed some comments and on-screen text to be more accurate