Basically every logging statement is wrapped by a nm_logging_enabled()
to evaluate the function call of the logging lazy.

Make the function a candidate for inlining, it safes some space. On
a default build it goes for me from 2580584 to 2560104 bytes (20k).

   $ ./autogen.sh && make && strip ./src/NetworkManager

We really expect this assertion not to be violoated.

As we want for nm_logging_enabled() to become smaller and inline,
remove the runtime assertion from regular builds.

Live fast and dangerous.

Instead of calling _ensure_initialized() at various places
to ensure that we setup logging before any logging commands
are executed, initialize the logging fields in the global variable.

This removes code from nm_logging_enabled(), which we want to
become a static inline function.

When one of the configuration snippet is malformed, NM doesn't tell
which file caused the error:

$ NetworkManager --print-config
Failed to read configuration: Key file does not start with a group

Fix this.

$ NetworkManager --print-config
Failed to read configuration: /usr/lib/NetworkManager/conf.d/test.conf: Key file does not start with a group

(fix wrong merge. I already cherry-picked 3d25b2e to nm-1-2)

https://bugzilla.redhat.com/show_bug.cgi?id=1337300

As explained in the function comment, a NULL value for @val is
acceptable. Otherwise:

$ nmcli connection edit
 (process:2276): nmcli-CRITICAL **: check_valid_name: assertion 'val' failed

Fixes: fb8fe1d8cb452d203c8cf0941c2a82e5903a0c8a

https://bugzilla.redhat.com/show_bug.cgi?id=1337300

VPN plugins are often not installed or they might be legacy-only.
In both cases we should show a better error message about the
failure reason.

Instead of using (only) a hard-coded list of VPN types,
prefer lookup the VPN settings from the .name files.

Still, fallback to a hard-coded list if the plugin cannot
be found, because for connection-add we currently don't
actually need the plugin installed.