Retrieve and validate rdev.license option.
Value
license string, one of c("mit", "gpl", "lgpl", "proprietary")
Details
rdev.license must be one of c("mit", "gpl", "lgpl", "proprietary"), and defaults to "mit".
If rdev.license is "proprietary", rdev.license.copyright (the name of the copyright
holder) must also be set.
