property_to_dbus() returns NULL when called with
NM_CONNECTION_SERIALIZE_WITH_SECRETS_AGENT_OWNED and the property is
not an agent-owned secrets. The function doesn't handle VPN secrets
correctly, since they are all stored as a hash in the vpn.secrets
property and the flag for each of them is a matching '*-flags' key in
the vpn.data property. VPN secrets must be handled differently; do it
in the VPN setting to_dbus_fcn() function.

Fixes: 71928a3e5cab ('settings: avoid cloning the connection to maintain agent-owned secrets')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/issues/230
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/merge_requests/280

Add test for checking the meta data for expected consistency.

This is also useful if you want to check something about the meta data
programatically.

For example, if you have the question which (if any) properties
are GObject based but also implement a to_dbus_fcn() function. Then you
can extend this code with some simple printf debugging to get a list of
those.

Or, if you want to find how many NMSettInfoProperty instances are in
static data (e.g. to determine how much memory is used). You can easily
modify this code to count them (and find 447 properties). Out of these,
326 are plain GObject based properties. Meaning, we could refactor the
code to create smaller NMSettInfoProperty instances for those, saving
thus (326 * 4 * sizeof (gpointer)) bytes (10K).

Such questions are interesting when refactoring the code.

  $ find * -type f |xargs perl contrib/scripts/spdx.pl
  $ git rm contrib/scripts/spdx.pl

No options are implemented yet.

At various places we only want to serialize agent-owned secrets. Without this
flag, we need to clone the setting first, then drop the secrets, then serialize
to D-Bus. Add a serialization flag to avoid that.

The name ("with") and the meaning of the flag is chosen in a way, that
there could be multiple such flags (NM_CONNECTION_SERIALIZE_WITH_SECRETS_NOT_REQUIRED),
and specifying at least one of them, would have the meaning to whitelist
flags of this kind. Specifying non of these "with" flags would have the
meaning of specifying *all*. Currently there is only one kind, so the name
and meaning is slightly counter intuitive.

  git ls-files -z -- ':(exclude)src/settings/plugins/keyfile/tests/keyfiles' | xargs -0 -n1 sed -i '1 { /^$/d }'

We no longer add these. If you use Emacs, configure it yourself.

Also, due to our "smart-tab" usage the editor anyway does a subpar
job handling our tabs. However, on the upside every user can choose
whatever tab-width he/she prefers. If "smart-tabs" are used properly
(like we do), every tab-width will work.

No manual changes, just ran commands:

    F=($(git grep -l -e '-\*-'))
    sed '1 { /\/\* *-\*-  *[mM]ode.*\*\/$/d }'     -i "${F[@]}"
    sed '1,4 { /^\(#\|--\|dnl\) *-\*- [mM]ode/d }' -i "${F[@]}"

Check remaining lines with:

    git grep -e '-\*-'

The ultimate purpose of this is to cleanup our files and eventually use
SPDX license identifiers. For that, first get rid of the boilerplate lines.

Merge the function pointer get_func() into to_dbus_fcn().

Previously, get_func() as handled separately from to_dbus_fnc()
(formerly synth_func()). The notion was that synth-func would syntetize
properties that are D-Bus only. But that distinction does not seem
very helpful to me.

Instaed, we want to convert a property to D-Bus. Period. The
implementation should be handled uniformly. Hence, now that is
all done by property_to_dbus().

Note that property_to_dbus() is also called as default implementation
for compare-property. At least, for properties that are backed by a
GObject property.

The naming was not very clear. How does get_func(), synth_func()
and to_dbus() relate? What does synth_func() do anyway?

Answers:

- get_func() and synth_func() do very similar. They should be merged
  in a next step.
  synth_func() has the notion of "synthetize" a property for
  D-Bus. As such, these properties are a bit unusual in that they
  don't have a backing GObject property in the setting. But it'd
  rather treat such properties like other properties. The step
  in that direction will be to merge the to-dbus functions.

- to_dbus() converts a GValue of the GObject property go GVariant.
  It's a simplified form of get_func()/synth_func() and a better name
  is gprop_to_dbus_fcn().
  The same for gprop_from_dbus_fcn().

For now, just rename.