Add: Support for build probability action CB 162 var10 0 (reverse rail vehicle)

[PeterN]

Implement support for OpenTTD's "vehicle build probability" callback 162.

This was intended to be a generic probability action, with the action provided in var 10. This is handled by nmlc.

The callback is implemented as 'reverse_build_probability`.

[andythenorth]

Works for me when tested with patched Horse.