tizen: Add additional unit for "unified" user session

tizen: Add additional unit for "unified" user session

The unified user session is about moving user session units, managed by
systemd --user, to main systemd, where it's managed as part of newly
created user@.target.

user@.target will contain same units as previously available in user/,
with same UID and environment setup.  systemd instance is used for unit
to be able to specify UID (inherited from user@.target).

The rationale behind this work is following:

* VD requirement to remove user session support
* boot time optimization requirements, due to:

   + 'systemd --user' taking 1s its own startup that could be used for
     unit startup
   + ability to better rearrange units if these managed by one systemd
     instance

Unit installed by this commit will not be used till user login mechanism
will be changed in systemd package (via changing pam_systemd to start
user@.target, rather than user@.service).

Change-Id: I62379f690efdb0b52bf22c671d5e7a42b6678e0e