Fix partial-read handling during restore

...by once and for all making all of the code deal appropriately with
expected partial reads.  We also now produce a properly conformant
underlying 'tar' EOF sequence [which will be compressed to almost
nothing] to doubly bulletproof the end-of-archive logic.

Fixes bug 5133658

Change-Id: I24a785574861d64ef10fc727b9f6b235575696b0