contrib/debian/control: allow any "man" virtual pkg (man-db/mandoc)

We support multiple "man" implementations; at least man-db's and
mandoc's.

So we can relax the mandoc dependency to a dependency on the virtual
package providing "man". Note that as of today, "mandoc" fails to
have a "Provides: man".

However since Debian policy says in
https://www.debian.org/doc/debian-policy/ch-relationships.html

> To specify which of a set of real packages should be the default
> to satisfy a particular dependency on a virtual package, list the
> real package as an alternative before the virtual one.

we want to list possible real packages anyway, so do that.

Closes #12596

contrib/debian/control

@@ -13,7 +13,7 @@ Build-Depends: debhelper-compat (= 13),
  python3-sphinx,
 # Test dependencies
  locales-all,
- man-db,
+ man-db | mandoc | man,
  python3
 # 4.6.2 is Debian 12/Ubuntu Noble 24.04; Ubuntu Jammy is 4.6.0.1
 Standards-Version: 4.6.2
@@ -26,8 +26,8 @@ Architecture: any
 # for col and lock
 Depends: bsdextrautils,
  file,
-# for man
- man-db,
+# for showing built-in help pages
+ man-db | mandoc | man,
 # for kill
  procps,
  python3 (>=3.5),