sd-bus: fix gvariant structure encoding

In gvariant, all fixed-size objects need to be sized a multiple of their
alignment. If a structure has only fixed-size members, it is required to
be fixed size itself. If you imagine a structure like (ty), you have an
8-byte member followed by an 1-byte member. Hence, the overall inner-size
is 9. The alignment of the object is 8, though. Therefore, the specs
mandates final padding after fixed-size structures, to make sure it's
sized a multiple of its alignment (=> 16).

On the gvariant decoder side, we already account for this in
bus_gvariant_get_size(), as we apply overall padding to the size of the
structure. Therefore, our decoder correctly skips such final padding when
parsing fixed-size structure.

On the gvariant encoder side, however, we don't account for this final
padding. This patch fixes the structure and dict-entry encoders to
properly place such padding at the end of non-uniform fixed-size
structures.

The problem can be easily seen by running:
    $ busctl --user monitor
and
    $ busctl call --user org.freedesktop.systemd1 / org.foobar foobar "(ty)" 777 8

The monitor will fail to parse the message and print an error. With this
patch applied, everything works fine again.

This patch also adds a bunch of test-cases to force non-uniform
structures with non-pre-aligned positions.

Thanks to Jan Alexander Steffens <jan.steffens@gmail.com> for spotting
this and narrowing it down to non-uniform gvariant structures. Fixes #597.