s4-ldb: make DN escaping/unescaping consistent

The DN escape function was using the form \c where c is any
character. The unescape function was using \XX where XX is a 2 digit
hex number. The asymmetry led to quite a few problems when we start to
deal with DNs containing escape chars, such as CN=foo\0ADEL:XXX. The
result was a DN that was not accessible.

This patch changes the escaping to follow RFC2253 much more
closely. We accept either type of escape, and produce the two types of
escape, depending on the character being escaped