Now nm_shutdown_wait_obj_*() supports two styles:

 - NM_SHUTDOWN_WAIT_TYPE_OBJECT: this just registers a weak pointer
   on a source GObject. As long as the object is not destroyed
   (and the object is not unregistered), the shutdown gets blocked.

 - now new is NM_SHUTDOWN_WAIT_TYPE_CANCELLABLE: this source object
   is a GCancellable, and during shutdown, the system will cancel
   the instances to notify about the shutdown. That aside, the GCancellable
   is tracked exactly like a regular NM_SHUTDOWN_WAIT_TYPE_OBJECT (meaning:
   a weak pointer is registered and shutdown gets delayed as long as the instance
   lives).

As the rest of the shutdown, it's not yet implemented on the shutdown-side.
What is now possible is to register such cancellables, so that users can make
use of this API before we fix shutdown. We cannot fix it all at the same time,
so first users must be ready for this approach.

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

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.

"nm-macros-interal.h" already includes <errno.h> and <string.h>.
No need to include it everywhere else too.

Extend nm_act_request_get_secrets() API to allow for the underlying
flexibility (of the API that it calls) to accept a strv list of hints.

Discovered by GCC 9:

  src/ppp/nm-ppp-manager.c: In function ‘_ppp_manager_start’:
  ./src/nm-logging.h:59:9: error: ‘%s’ directive argument is null [-Werror=format-overflow=]

Fixes: 35d9169c3c4a0361c363eca8a34ed6c575a620f1

Refactor some code to use nm_streq() and NM_IN_STRSET() instead of
strcmp().

Note that nm_utils_get_ip_config_method() never returns %NULL (not even
with g_return*() assertion failures). nm_streq() is sufficent.

Recently, more and more code was refactored to use an addr_family
integer to distinguish between IPv4 and IPv6.

Refactor nm_utils_get_ip_config_method() and nm_device_get_effective_ip_config_method()
to do that too. If we use different identifiers, we need to translate from one to
another and its inconsistent. Also, accessing a GType is an unnecessary function call,
instead of a plain constant.