frontend stuff moved into a database structure, essentially to allow overlays to be processed before database selection; passes all tests (ITS#3080)

misc other cleanup

(slapi_x_XXX is still reserved for exported functions that are not part
of the SLAPI specification)

The oblique SLAPI interface requires that preop bind plugins return 1 in the
case that they have authoritatively handled the bind; of course, this is
the same return value that doPluginFNs() returns in the case of no plugins
being called.

This patch should distinguish between the two cases...

Other Cleanup

order to prevent the operation from being passed to the backend. It
need only return a non-zero error code.

consolidated into the Operation structure. All reply parameters
are consolidated into the new SlapReply structure. Most operations
now have identical call signatures... Changes are not #ifdef'd,
revert to -r NO_SLAP_OP_BLOCKS if necessary to back out.