meson: allow extra net naming schemes to be defined during configuration

meson: allow extra net naming schemes to be defined during configuration

In upstream, we have a linearly-growing list of net-naming-scheme defines;
we add a new one for every release where we make user-visible changes to the
naming scheme.

But the general idea was that downstream distributions could define their
own combinations (or even just their own names for existing combinations),
so provide stability for their users. So far this required patching of the
netif-naming-scheme.c and .h files to add the new lines.

With this patch, patching is not required:

$ meson configure build \
  -Dextra-net-naming-schemes=gargoyle=v238+npar_ari+allow_rerenames,gargoyle2=gargoyle+nspawn_long_hash \
  -Ddefault-net-naming-scheme=gargoyle2

or even

$ meson configure build \
  -Dextra-net-naming-schemes=gargoyle=v238+npar_ari+allow_rerenames,gargoyle2=gargoyle+nspawn_long_hash,latest=v249 \
  -Ddefault-net-naming-scheme=gargoyle2

The syntax is a comma-separated list of NAME=name+name+…
This syntax is a bit scary, but any typos result in compilation errors,
so I think it should be OK in practice.

With this approach, we don't allow users to define arbitrary combinations:
what is allowed is still defined at compilation time, so it's up to the
distribution maintainers to provide reasonable combinations. In this regard,
the only difference from status quo is that it's much easier to do (and harder
to do incorrectly, for example by forgetting to add a name to one of the
maps).