ITS#9363 Zero out value on config delete
This lets back-config reset values for non-magic ARG_TYPES values to zero.
There are 89 cases of this across openldap+contrib, most of them are ok (default is 0/NULL or the value is a MUST), the following have a non-zero default and would be affected:
- back-sql/ndb fields
- index_substr_any_*
- localSSF
- sockbuf_max_incoming*
- rtxnsize (in back-mdb)
- sssvlv-maxkeys and sssvlv-maxperconn
All of the above could be fixed by adding a default parameter in the config table, maybe dropping the notify field there while we're at it, since it's never been implemented. That's what I intend to do here in the next version of this MR.