1.6.1

Improvements since 1.6.0:

- #101:
    - Enhance portability of unit tests.
    - Disable misleading unit test error messages.
- #102: Upgrade `autogen.sh` so the `configure` script can also be
  easily generated in OpenBSD and FreeBSD.
- #103:
    - Remove useless logging messages during startup.
    - Stop printing logging severity (`INF`, `WRN`, `ERR`) in syslog.
    - 31414cd2ab6634c53a5ca31531674a82bf778d1c: Rephrase HTTP GET logs.
    - Remove false alarm error message at the end of validation cycle
      logs:

> Cannot generate [rsync URL]'s cage. I'm probably going to end up
> deleting it from the cache.

- #104: Add `CACHEDIR.TAG` file to cache, to hint backup software not to
  synchronize it. (See https://bford.info/cachedir/)
- ec918adeb0e641d41ee2571fd717cad139d939ad: Remove bug-induced Cache
  Resets during RTR communication.
- 161c2af306d1e2ea87cfe540f1b69770ff84bd8d: Automatically clean up cache
  whenever Fort's version changes.
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