fix(Makefile): Copy systemd units recursively.

Needed to install things like dropin and wants directories.
fix(systemd): Use a dropin for sshd service ordering.

Setting Before=sshd@.service in sshd-keygen does not actually work.
Instead install a dropin to configure the dependency from sshd@.service

Prune some extra cruft, no need for logging to the console or an
[Install] section in sshd-keygen.service.
chore(systemd): Install wants symlinks with units.

Now that the system directory is coppied recursively we can install the
symlinks directly rather than bothering with systemd_enable_service in
the ebuild.
feat(systemd): Add service to create nsswitch.conf

systemd-tempfiles cannot be used to initialize a completely empty /etc
directory right now because it needs to look up users and groups. Until
a more complete long-term solution is completed on the systemd side of
things for this situation this little one shot will fix this up.

We will need to keep our eye out for similar issues that may be lurking.
fix(systemd): Never remount / or /usr in containers
fix(systemd): Don't call ldconfig before creating /etc/ld.so.conf
Merge pull request #111 from marineam/units

Unit updates