\n is automatically converted on write to the platform-dependent os.linesep.
Using os.linesep instead of \n means that on Windows, the line ending becomes "\r\r\n".
Also switches mentions of {LF} to {NL} in code and docs.
export-tar: just msgpack and b64encode all item metadata and
put that into a BORG specific PAX header.
this is *additional* to the standard tar metadata.
import-tar: when detecting the BORG specific PAX header, just get
all metadata from there (and ignore the standard tar
metadata).
