Add AD control support and fix Unknown control variant (#71)

Add SearchOptions, ShowDeleted, SdFlags, and ExtendedDn control
variants for Active Directory interoperability. Preserve criticality
and value bytes in the Unknown variant so unrecognized controls
round-trip without data loss. Fix 2-element control sequence parsing
to distinguish criticality from value by inspecting the BER tag type.

Author: danielsreichenbach