btrfs-progs: optionally restore metadata

Changes since v1:

* Documented in the manpage
* Added to usage() for btrfs restore
* Made it an optional flag (-m/--restore-metadata)
* Use endian-safe macros to access the on-disk data.
* Restore the proper mtime instead of atime twice.
* Restore owner and mode
* Restore metadata for directories as well as plain files.
* Since it's now explicitly requested, errors are fatal unless ignore_errors is requested.

I tested this on the array I'm restoring, it looks sane to me.

Thanks to Noah Massey for the patch review, and Duncan for the prompt to add owner/permissions to the patch.

Symlinks and hardlinks are beyond the scope of these changes, I'll look into them if this looks good to everyone.

That would be appreciated. I'll send you a POC code to dump the symlinks, it's missing the actual file creation and commandline interface etc.