Modify Add/Delete/Modrdn operations to require write perms on

Modify Add/Delete/Modrdn operations to require write perms on
parent's "children" attribute.  Write lock parent to prevent
multiple clients making conflicting operations concurrently.
If parent doesn't exist (ie: is backend root), acquire a writer
lock (a simple mutex) on the "root."
Use -DSLAPD_CHILD_MODIFICATION_WITH_ENTRY_ACL to use the child's
"entry" acl for modrdn/delete operations.