Apply directory metadata only after the directory was populated

[folbricht]

Fixes #376.

desync untar failed with Error: <dir>: mkdirat <dir>: permission denied (or openat <file>: permission denied) when the archive contained a read-only directory that holds further directories or files:

mkdir -p test/test
chmod -R 0555 test
desync tar test.catar test
desync untar test.catar test2

LocalFS.CreateDir() applied the mode from the archive immediately after creating the directory, making it impossible to write the contents that follow.

Directory metadata (owner, xattrs, permissions and timestamps) is now deferred until the directory has been fully populated, which is the case when an entry outside of it arrives, or when the archive ends. Directories that aren't writable and searchable by the current user are temporarily given those permissions while their contents are being written, the same way GNU tar handles this. UnTar() signals the end of the archive through an optional Finalize() method, so the other FilesystemWriter implementations (tar, mtree) are unaffected. LocalFS.Close() applies whatever is still outstanding on a best-effort basis, for the case an extraction is aborted.

Deferring the timestamps fixes a second issue found while testing this: directory mtimes were not preserved since writing the contents of a directory updates its mtime. GNU tar delays this for the same reason. Extracting a tree with directories dated 2020 now gives the same result as tar xp rather than dating them to the time of extraction.