s4:s3compat Add pdb_samba4

s4:s3compat Add pdb_samba4

pdb_samba4 is a passdb module based on the work Volker did on pdb_ads,
but written directly against the LDB and DSDB APIs used in the source4
code.

In particular, the SID-based extended DN and dsdb_ helper functions
are used to make this a much smaller module.  It contains some write
support, but as the first customer is winbindd, this is unfinished.

The idmap fucntions link to Samba4's idmap API

Andrew Bartlett