fix(sys-apps/dbus): Add slot dependency on systemd

This will force a recompile of dbus when systemd's sub-slot changes as
it did from 207->209 when some lib versions changed.
clean(sys-apps/dbus): Remove unused ebuilds
fix(sys-apps/baselayout): Use custom script to generate /etc/group

I misunderstood the documentation for systemd-tmpfiles, if a string to
write to a file is provided it will always write it, even if the file
already exists and doesn't need to be created. This means that using
tmpfiles to initialize /etc/group results appending each boot.

Instead use a little script instead, also initialize passwd and shadow
so that the `passwd` command works for the core user.

Make use of the systemd eclass where applicable.
fix(profiles): Add which to stage1 bootstrap.

libseccomp calls which in its ./configure script so when building stage3
if which isn't already there the build will fail. This is quick fix.

Reported upstream: https://bugs.gentoo.org/show_bug.cgi?id=502084
Merge pull request #378 from marineam/misc

misc updates