- Drop systemd's dependency on udev (jsc#PM-2677)
In some environments (i.e. containers) udev is usually not necessary
but pulls in unnecessary packages.

- Now that chkconfig/insserv are history, let's implement the strict
minimum in systemd-sysv-install to enable/disable SysV init scripts
(bsc#1186595 bsc#1186359)
Indeed there's no much point in dropping SysV support completely
until upstream will do especially since 3rd party applications such
as vmware still rely on it, see bsc#1186359).

- Allow the sysusers config files shipped by systemd rpms to be
overriden during system installation (bsc#1171962)
- While at it, add a comment to explain why we don't use
%sysusers_create in %pre and why it should be safe in %post.

- udev requires systemd in its %post (bsc#1185958)
udevadm, called in udev's %post, requires libsystemd-shared-248.so.

- Restore all "License:" tags
udev uses a different license (GPL-2.0-only) than the main package
and "osc service localrun format_spec_file" has the good taste to
restore the license tags for all other subpackages if one of the
subpackage tag differs.

- Expect 644 permissions for /usr/lib/udev/compat-symlink-generation (bsc#1185807)

- Spec file minor cleanups:
- Drop all "Group:" tags as they are deprecated.
- Drop "License:" tags from all subpackages and make it inherited