back-config OIDs

[Hallvard B Furuseth <h.b.furuseth@usit.uio.no>]

Attribute (OLcfgDbAt:0.1 NAME 'olcDbDirectory') has different
definitions in back-ldif and back-bdb: No SINGLE-VALUE in back-ldif.

Looks like a likely bug to reappear when other "common" attributes are
added.  I suggest that OIDs in the "common" namespace are defined in
bconfig.c (in this case with no way to configure the attribute), and
that backends that use them put just the OID or attribute name instead
of the whole attribute definition in ConfigTable.attribute.

Currently that would also apply to:
  back-bdb:  (OLcfgDbAt:0.2 'olcDbIndex'), (OLcfgDbAt:0.3 'olcDbMode')
  back-ldap: (OLcfgDbAt:0.14 'olcDbURI')

-- 
Hallvard