Prepare COMMIT_MSG for import to pkgsrc.

Remove MESSAGE.  This repeats pkgsrc-wide configuration norms and
exhorts to read the upstream documentation.

Install upstream's postinstall as documentation.

Fix path to examples.

Fix detection of libevent2.

This package has known vulnerabilities, please investigate and fix if possible:
  CVE-2018-4056, CVE-2018-4058, CVE-2018-4059, CVE-2020-4067, CVE-2020-6061,
  CVE-2020-6062, CVE-2020-26262
