chlog,cli,docs: offer suite-aliased series in the chlog selector

The chlog series selector only appeared when the changelog's current
distribution resolved to a known series; a Debian package targeting
'unstable' (or 'stable', 'testing', ...) fell through to keeping the
current series, silently, with no menu.

Resolve the changelog distribution through the suite aliases first
(unstable identifies the same series as sid, which resolves to the
Debian series list). The selector offers an aliased series as
'<suite> (<series>)' — 'unstable (sid)' — preselected, but selects
the suite name: what a changelog distribution field expects, instead
of the codename. Every other label and free-typed input selects
itself, unchanged.

Reflect the selector in the README roadmap checklist.

README.md

@@ -115,6 +115,7 @@ Missing features:
 - [ ] `pkh chlog`
   - [x] Auto-generate changelog entry
   - [x] Extra flags: backport, non-maintainer upload, no change rebuild, ...
+  - [x] Select the target series, matching changelog suite names with their series (unstable ≡ sid)
   - [ ] Commit changelog entry
 - [x] `pkh new`
   - [x] Scaffold a new Debian source package (interactive, multiple languages)