Run yum, emerge, port, and openbsd backends on real systems

[trmxvibs]

Context

v0.2.0 added five backends written from official documentation. All five
have been checked against upstream docs — two bugs were found and fixed
along the way (choco's removed --local-only flag, emerge's dangerous
--unmerge). But four have never been run on the systems they target:

Backend	Target OS	File
yum	RHEL 7, older CentOS, slim container images	src/pkgwrap/backends/yum_backend.py
emerge	Gentoo	src/pkgwrap/backends/gentoo_backend.py
port	macOS with MacPorts	src/pkgwrap/backends/macports_backend.py
openbsd	OpenBSD, NetBSD	src/pkgwrap/backends/openbsd_backend.py

(winget was confirmed on real Windows in #9 follow-up testing — see
CHANGELOG for the fix that came out of it.)

How to help

You do not need to install or change anything. --dry-run prints the
command without executing it:

pkgwrap install curl --dry-run
pkgwrap remove curl --dry-run
pkgwrap refresh --dry-run
pkgwrap upgrade --dry-run
pkgwrap search curl --dry-run
pkgwrap list --dry-run
pkgwrap info curl --dry-run
pkgwrap clean --dry-run

Then comment with your OS/version, pkgwrap --backend output, the commands
above, and whether each one is what you'd actually type by hand.

If you want to go further, run the non-destructive ones for real (search,
list, info) and report what happens.

Open questions

• port: does port clean --all installed do what a user expects from a
  clean command, or is it too aggressive?
• openbsd: is there a sensible metadata-refresh equivalent, or is
  reporting refresh as unsupported correct?
• yum: yum check-update exits with status 100 when updates exist —
  confirmed makecache is the safer refresh, but worth a second opinion.

Acceptance criteria

• Each of the four backends has at least one confirmation from a real system
• Any incorrect commands are fixed, with a test covering the corrected shape
• README "Verification status" note is updated as each backend is confirmed