Merge maint into master after the v0.4.1 release

Carries the v0.4.1 content up the one-way branch flow. The release itself
belongs to maint, so the parts of this merge that identify a version are
resolved in master's favor and the parts that describe the project's state
are taken from maint.

Kept master's: Cargo.toml and Cargo.lock at 0.5.0-dev, the README status
badge, and the "FIPS is at v0.5.0-dev on the master branch" line. A patch
release consumes no minor version and master's development line is
unaffected by it.

Took from maint: the bloom antipoison FPR default change and the
duplicate-definition fix it rests on, the two docs that describe them, the
new v0.4.1 release notes, the correction to the v0.4.0 release date, and the
[0.4.1] changelog section, which slots below master's own [Unreleased] and
above [0.4.0]. The root RELEASE-NOTES.md mirror moves to v0.4.1 because it
tracks the latest shipped release, which also clears the stale provisional
date it had been carrying.

One line needed splitting rather than choosing: the README said "v0.4.0 has
shipped" inside the paragraph that identifies master as 0.5.0-dev. The
version identity stays master's and the shipped-release pointer moves to
v0.4.1.

Quartet green on the result: 1645 tests passed, clippy clean with -D warnings.

README.md

@@ -213,7 +213,7 @@ testing/      Docker-based integration test harnesses + chaos simulation
 ## Status & roadmap
 
 FIPS is at **v0.5.0-dev** on the `master` branch.
-[v0.4.0](https://github.com/jmcorgan/fips/releases/tag/v0.4.0) has
+[v0.4.1](https://github.com/jmcorgan/fips/releases/tag/v0.4.1) has
 shipped; this development line continues the testing-and-polishing
 track toward v0.5.0. The core protocol works end-to-end over
 UDP, TCP, Ethernet, Tor, Nym, and Bluetooth on a global, public test