Check empty cachedir for -d, --print-stats, --recover

[gperciva]

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[gperciva]

I'm not entirely sold on this. On one hand, it does the desired:

$ ./tarsnap --cachedir empty-cachedir/ -d -f tiny
tarsnap: Cache directory must already exist for -d; check --cachedir and/or run --fsck

On the other hand, it seems messy to #include "chunks.h" in bsdtar.c.

On the gripping hand, glue.c doesn't seem much better. And while I could certainly add functions to multitape etc. to pass the function call down the layers, that seems like overkill. Another option would be to move chunks_directory_exists() to a separate chunks_really_public.h, then only include that one in bsdtar.c.

So this seems to me like the least bad way to implement this, but I don't like it.