Issue description:
in C dn2str() supports flags which works by providing LDAP_DN_FORMAT_UFN, LDAP_DN_FORMAT_AD_CANONICAL. These symbols do exist in Python, but cannot be used ultimately because the Python counterpart is pure Python and does not pass to dn2str(3). It'd be great if this would work as in C for dn2str and str2dn.

Operating system: FreeBSD 11.2-STABLE

Python version: 3.6.8

python-ldap version: 3.1.0