devel/libevent: Update to 2.1.12

Update devel/libevent to 2.1.12 [1]
Change to use the dist archives instead of the tarball provided by GitHub,
per notice from upstream developers. [2] [3]
Add CPE information [2]

[3]: https://github.com/libevent/libevent/releases/tag/release-2.1.12-stable
Changelog:
https://raw.githubusercontent.com/libevent/libevent/release-2.1.11-stable/ChangeLog

PR: 247985 [1]
Reported by: Yasuhiro KIMURA [1]
Suggested by: daniel.engberg.lists pyret net [2]
MFH: 2020Q3