let req_modify_s and req_modrdn_s have a common portion to interoperate within slap_mods_opattrs() (ITS#5093)

(ConfigReply) for the error messages.

_db_destroy functions.

major improvement: baseline support for back-config; minor improvement: trim code to handle absence of librewrite (enable forced when slapo-rwm is configured...)

undefined mapped attribute types.

(this is preliminary, more work remains)

#unifdef -DSLAP_RELAX

let overlays define if they must appear exactly once in a chain, and if they must be global/per-db only, and holler in case

this to the searchDN context. Please review.

ITS#3914 bug was very close to that leak: rwm_dnattr_rewrite() already freed the original value (all this stuff may need work)