Allow admin to specify the startup data variable name

[robertopreste]

From #560 (review), the Admin user should be allowed to specify which environment variable to use to open a file at application startup. Currently, this is hard-coded to be SRDATA, but depending on users' needs, this could be something else (e.g. MY_DATA or APP_DATA).
An option in the Users policies section should be exposed to set this variable name.