If we're a single-user installation of Nix, then we won't have root
superpowers to just ignore the permission bits. This means that we'll
need permission on the directory (if it's a directory) that we're moving
in order to move it with rename, because it must update the ".."
directory entry.

Fixes #4295.

This is already working in master, thanks to
e913a29 (and followup cleanups).

When moving paths out of sandbox, ensure we have write permission

This fixes results for arm64 macOS so config.guess now reports:

  aarch64-apple-darwin20.1.0

instead of

  arm-apple-darwin20.1.0

(cherry picked from commit 94f3595)

As fromTOML supports \u and \U escapes, bring fromJSON on par. As JSON defaults
to UTF-8 encoding (every JSON parser must support UTF-8), this change parses the
`\u hex hex hex hex` sequence (\u followed by 4 hexadecimal digits) into an
UTF-8 representation.

Add a test to verify correct parsing, using all escape sequences from json.org.

Backport json unicode escaping to 2.3

Allow HTTP binary cache to request absolute uris