Working in "blk/" folder from a modern system is harder than it
should be. Moving blocks around is a bit awkward, grepping is a
bit less convenient than it could be, git blame has troubles
following, etc.

In this commit, we modify blkpack and blkunpack to work with single
text files with blocks being separated by a special markup.

I think this will make the code significantly more convenient to
work into.

* Report when a file cannot be opened

* Close files when done reading them

The man page says it's not always available and it caused problems
under some FSes. The condition is not needed in the context of
blk/, let's scrap it.