Rewrite documentation for `Distribution.License`.

* Fix copyright field.
* List valid versions for each license.
* Remove descriptions for most licenses; just link to them instead.
* Explain why BSD4 should be avoided.
* State that public domain is not a license.
* Link to user guide.
Eliminate redundant wording.
Merge branch 'license-docs' of https://github.com/ianbollinger/cabal
Add support for 'main-is' to cabal init.

This includes a new flag, --main-is, as well as a new prompt that appears if the user selects the 'Executable' option or uses --is-executable.
Address review comments.