Append with EXTHDR fix, tests and documentation improvements

* Fixed bug for trying to append non-existient entry with `EXTHDR` flag:  
Now we treat appends of non-existient entries as new writes thus ignoring `APPEND` flag. But we preallocate space aggressively in that case;
* Documentation fixes: `s/ioremap/reverbrain/g`;
* Improved tests;
* Added more sanity to empty blob removal path, just in case...