I) Filename was not put in temp Main generated to save selected data only,
this was breaking readcode when trying to open partial file, leading to missing
filename in final loaded Main data.
II) Read code would confuse partial .blend files with Undo ones, when they had no screen in them
(which happens to 99.999% of partial .blend files I guess).
Reported by @sybren, thanks.
Should be safe enough for 2.78 release.
When writing temp blenbuffer file, libraries of linked datablocks where not tagged correctly, which
means they were not put in the temp Main used to write the buffer, resulting in implicit localization
of linked data.
