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 1e86730a)