A build from scratch and a rebuild on a cached tree install the same programs

[DevomB]

What was wrong. Stage 04 builds its steps in a fixed order, but some packages pick up an optional library only if it is already installed. A build from scratch, as a release tag's is, sees only the steps before it. A rebuild on a cached tree sees everything an earlier build installed. Two packages came out different:

• iproute2 builds before libmnl. From scratch it built without libmnl. On a cached tree it found libmnl and built seven more programs (devlink, rdma, tipc, dcb, vdpa, dpll, netshaper), plus an ip that prints the kernel's extended error messages.
• Python builds before gdbm. On a cached tree it built _gdbm too.

The program list (#403) was made from a cached tree's image. A build from scratch then failed stage 06 on eight entries naming nothing (Distro 38246073914). This is the trap perl's recipe already guards against for its GDBM_File module.

What changed.

• libmnl is built right before iproute2 instead of with the net zone's packages. nftables, which comes later, still finds it.
• iproute2's Makefile no longer lists the libmnl tools; nothing here runs them. The recipe fails if a later iproute2 lists them another way. It also removes their man pages and completions, which install regardless, and any copies a cached tree holds.
• ip and tc now always build with libmnl, so a failed ip command in the net zone's log says what the kernel refused, for example "Error: Nexthop has invalid gateway." instead of only "RTNETLINK answers: Invalid argument".
• Python is configured without _gdbm and _dbm (py_cv_module__gdbm=n/a, py_cv_module__dbm=n/a), and the recipe removes any copy a cached tree holds. Nothing imports dbm.gnu or dbm.ndbm; dbm itself falls back to dbm.dumb.

How the run proves it.

• Stage 06 and the acceptance refuse an executable file build/config/image-programs.txt does not name #403 now drops the eight entries (05204bc), so its list names what a build from scratch installs.
• The second run of ready-fold-09 restores the first run's stage 04 tree and rebuilds from Python on, which is exactly the cached-tree case. Stage 06's program check then passes only if that rebuild installs neither the seven tools nor _gdbm.
• The zones suite runs the net zone's ip and nft on the result.

Stacked on #348, whose lines in python.sh this extends. Merge it with #403.