Store AppArmor label of connecting processes

Store AppArmor label of connecting processes

When processes connect the bus, the AppArmor confinement context should
be stored for later use when checks are to be done during message
sending/receiving, acquire a name, and eavesdropping.

Code outside of apparmor.c will need to initialize and unreference the
confinement context, so bus_apparmor_confinement_unref() can no longer
be a static function.

[Move bus_apparmor_confinement_unref back to its old location for
a more reasonable diff -smcv]

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=75113
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>