pyldb: Split text/byte strings for compatibility with Python 3

pyldb: Split text/byte strings for compatibility with Python 3

Compatibility with Python 2, and backwards compatibility on Python 2,
is kept.

Under Python 3, DNs, attribute names, filters, controls are always text
(unicode) strings, encoded to/from UTF-8 for storage.
Attribute values are byte strings.

When creating DNs and attribute values, both text and bytes are accepted.
This allows creating messages from homogeneous dicts.

LDB Messages and MessageElements have a .text attribute, which offers
a text view on the contents: any value retrieved from it will be a text
string. The wrapper is implemented in a new Python module.

Thanks to Stefan Metzmacher for const warning fixes

Signed-off-by: Petr Viktorin <pviktori@redhat.com>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>