Similarly as in spa_dbus, don't exit if bus goes down. Neither the
audio or midi Bluetooth backend reconnects to DBus, but they shouldn't
exit the process.
Use stdalign.h instead of union.
Fix some refcounting and return values.
Fail early in add_filters.
Minor style cleanups.
Less magical spa_dbus_async_call.
As server, put node nick in GATT Characteristic User Description, so
that clients have some information which endpoint is which in case we
have multiple. This is not in BLE MIDI standard, but cannot hurt.
As client, make use of that information, if present.
