This has several benefits:
* User count remains coherent, regardless of the order in which you use 'user_one' & real refcounting users
(i.e. if you add to group, and then link in scene, or the reverse, you now always get same final user count).
* Avoids the need to check for potential 'user_one' extra user in count in several places in code (e.g. when
making IDs single users...).
* Users won't wonder why they cannot make 'single user' and ID even though its user count shows '2'!
* readfile.c now always uses code from BKE's library.c when modifying id->us. Which means we can consider
(asside from assignment during initialization) that id->us is read-only outside of library.c context.
Note that this commit reverts previous one (rB6b1d77a8052b) - please **do not** backport this one in 2.77.
00166ff62e