Add support for SD_BUS_DEFAULT*

Add support for SD_BUS_DEFAULT*

Currently, sd-bus supports the ability to have thread-local default busses.
However, this is less useful than it can be since all functions which
require an sd_bus* as input require the caller to pass it. This patch adds
a new macro which allows the developer to pass a constant SD_BUS_DEFAULT,
SD_BUS_DEFAULT_USER or SD_BUS_DEFAULT_SYSTEM instead. This reduces work for
the caller.

For example:

    r = sd_bus_default(&bus);
    r = sd_bus_call_method(bus, ...);
    sd_bus_unref(bus);

Becomes:

    r = sd_bus_call_method(SD_BUS_DEFAULT, ...);

If the specified thread-local default bus does not exist, the function
calls will return -ENOPKG. No bus will ever be implicitly created.