chlog: number entries as backport, NMU or no-change rebuild

The version flags were never reachable: --backport was declared but not
read, and compute_new_version's NMU/rebuild numbering sat behind a TODO
asking for CLI wiring (the old positional-bool signature always received
false). generate_entry now takes an EntryKind selected by three mutually
exclusive flags, and is async because the backport numbering derives the
Debian release number of the target series from distro-info:

- --backport: 1.0-1 becomes 1.0-1~bpo12+1 (12 = release number of the
  target series, backport suite names accepted too). Re-running on an
  already-numbered version bumps the counter; series without a numeric
  Debian release (sid, Ubuntu series, UNRELEASED) are rejected before
  anything is written.
- --nmu: 1.0-1 becomes 1.0-1.1 (native 1.0 becomes 1.0+nmu1).
- --rebuild: 1.0-1 becomes 1.0-1build1.

An explicit --version overrides all three. compute_new_version went
from four positional bools to a private Bump enum; backport numbering
reuses increment_suffix with a '~bpoNN+' suffix. The CLI prints the
computed new version before opening the editor.

README.md

@@ -90,7 +90,7 @@ Missing features:
   - [ ] Try to fetch the correct git branch for series on Debian, or fallback to the archive
 - [ ] `pkh chlog`
   - [x] Auto-generate changelog entry
-  - [ ] Extra flags: backport, non-maintainer upload, no change rebuild, ...
+  - [x] Extra flags: backport, non-maintainer upload, no change rebuild, ...
   - [ ] Commit changelog entry
 - [ ] `pkh build`
   - [x] Build the source package