s4:ldb: add infrastructure for extended dn handlers

s4:ldb: add infrastructure for extended dn handlers

This introduces a new set of pluggable syntax, for use on the
extended DN, and uses them when parsing the DN.

If the DN appears to be in the extended form, we no longer return the
full DN 'as is', but only return the normal part from
ldb_dn_get_linearized().

When validating/parsing the DN we validate not only the format of the
DN, but also the contents of the GUID or SID (to ensure they are
plausable).

We also have functions to set and get the extended components on the DN.

For now, extended_dn_get_linearized() returns a newly constructed and
allocated string each time.

Signed-off-by: Stefan Metzmacher <metze@samba.org>