fix DN_SEPARATOR() and clarify its use

servers/slapd/acl.c

servers/slapd/aclparse.c

servers/slapd/back-bdb/cache.c

servers/slapd/back-ldap/suffixmassage.c

servers/slapd/back-meta/search.c

servers/slapd/backend.c

servers/slapd/dn.c

servers/slapd/limits.c

servers/slapd/overlays/rwmdn.c

servers/slapd/slap.h