audit-manager: enable logging.audit by default when libaudit support is available

audit-manager: enable logging.audit by default when libaudit support is available

Previously, when compiling NetworkManager with libaudit support, it
was disabled by default and only used after setting logging.audit=true.

Turn that around. If we compile NetworkManager with audit support, we also
enable it by default. The user can then explicitly disable it by
configuring logging.audit in NetworkManager.conf.

But also, add a configure option 'yes-disabled-by-default' to compile
with audit support, but have it disabled by default. This would be the
previous behavior, but it must be enabled explicitly.

Fixes: be49a59fb649d7383b456403deb323a2eb9dab4b