Most subsystems are now created via the subsystem manager using the global
subsystem registrations.
The FGGlobals add_subsystem() and add_new_subsystem() methods have been removed
as the subsystem manager addition and creation function interface now exceeds
the functionality of these helper functions.
It was supposed to be in ft/min, but everything except AIAircraft was
using it as ft/sec. Change the name to AIBase::vs_fps and ensure the
same unit is used everywhere.
