This commits deletes all references and code for back-bdb and back-hdb.
There is some follow up work still necessary to flush out the admin
guide for back-mdb.

Other documentation updates to make it clear that (L)MDB is preferred

MDB is the name of the backend, LMDB is the name of the DB engine.
Also update some BerkeleyDB info.

* Align INSTALL and doc/guide/release/install.sdf, so
  INSTALL = (sdf -2txt doc/guide/release/install.sdf) + copyright.
* Autoconf recommends './configure var=val' over 'var=val ./configure'.
* Add -W,-rpath,<path> to an LDFLAGS example.

is more accurate; slapdconfig.sdf should eventually be dropped.

Refer to README instead.