Renaming refused on hosted drives because encrypted names and contents are
bound to their path. The encrypting client now renames itself: readable
files keep s3d's server-side copy, the rest are decrypted and encrypted again
for their new place. A drive with shared folders or links is not renamed,
since its rules belong to the old name.
Free space is forwarded to the server, whose s3d answers for its own
folder, instead of reporting nothing. space.js now builds on sia-account.js,
so BLAKE2b and the indexer request signing exist once.
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