Like the previous commit. Move code that depends on libnm-core out
of shared to avoid circular dependency.
Also add a readme file explaining the reason for existence of
the helper libraries nm-libnm-core-intern and nm-libnm-core-aux.
The "shared" directory is used by libnm-core, it should thus only depend on
code that is in the "shared" directory. Otherwise there is a circular
dependency, and meson's subdir() does not work nicely.
Also, libnm-core is really part of (and also an extension of) libnm-core,
so it belongs there.
I guess, the original idea was that this is also an extension for libnm,
so another project could take these utility functions (by copying them
into their source tree) and use them. That is still possible, it's
just that the sources are no longer under the shared directory.
Also add a readme to explain the non-obvious meaning of these files.
