gnu: libevent: Update to 2.1.8 [security fixes].

* gnu/packages/libevent.scm (libevent): Update to 2.1.8.
[inputs]: Change 'python-wrapper' to 'python-2'. Move 'which' to ...
[native-inputs]: ... here. New field.
(libevent-2.0): New variable.
* gnu/packages/patches/libevent-2.1-dns-tests.patch,
  gnu/packages/patches/libevent-2.0-evdns-fix-remote-stack-overread.patch
  gnu/packages/patches/libevent-2.0-evdns-fix-searching-empty-hostnames.patch
  gnu/packages/patches/libevent-2.0-evutil-fix-buffer-overflow.patch: New files.
* gnu/local.mk (dist_patch_DATA): Add them.
* gnu/packages/gnuzilla.scm (icecat)[inputs]: Change 'libevent' to 'libevent-2.0'.