util/__init__.py: enable unicode_literals

Unicode literals are nice for uniformity, but in this file it's more
complex than usual, since Python 2's shlex does not support unicode.
So, use _native_string() for compatibility in this case.
Update notes about aliases, eclass-overrides and masters attributes in repos.conf and layout.conf.
Print error when section name set in repos.conf differs from name set inside repository.