Allow acquired aircraft -> storage. Allow aircraft with expired checks on UM

[JumboShrimp]

Main reason to avoid triggering automatic C or D check on arrival of the aircraft or removal from UM listing.

Also, allow placing aircraft on UM with expired C/D checks and / or leave it UM after C / D check expires.

Both options will make it possible to buy and sell aircraft intended for conversions (to combi or cargo).

Also, seller will not have to pay for performing of C/D checks just to maintain the availability of aircraft to be for sale.

[Fisher970]

I wouldn't mind a "convert to (combi/freighter/etc) at next D-check" option as well

[Sami]

The system was once changed to only allow planes with valid C/D to be listed at the used market, so we'll keep that part the way it is currently.