Conform BSD-2-Clause and BSD-3-Clause text to SPDX
`cabal init` text for BSD-2-Clause and BSD-3-Clause licence differed
slightly from the one published at SPDX. [1] [2]
This caused some problems to users when dealing with licence-recognition
software. [3]
[1] https://spdx.org/licenses/BSD-2-Clause.html
[2] https://spdx.org/licenses/BSD-3-Clause.html
[3] https://discourse.haskell.org/t/non-standard-license-generated-by-stack-new/9059
(cherry picked from commit
1e86730a75d1fbba01d87cb68f11999ac8b5302c)