For consistency with other options, and because there's already some code in
the wild using dashes instead of underscores.

If people don't write stupid code like 'if result is True:',
this small API change should not break anything.

Add new index() method to add an entry to shadir when the data is known to be
already in shacache.

When uploading big file, this avoids trashing the kernel cache once more.

Those helpers cover most of slapgrid use cases: download AND upload from/to
string (fd), file, directory.

Fetch all trustable entries and let client decide which entries to use.