StartTLS (ITS#3507) + chain overlay fixes and improvements

servers/slapd/back-ldap/add.c

servers/slapd/back-ldap/back-ldap.h

servers/slapd/back-ldap/bind.c

servers/slapd/back-ldap/chain.c

servers/slapd/back-ldap/compare.c

servers/slapd/back-ldap/config.c

servers/slapd/back-ldap/delete.c

servers/slapd/back-ldap/extended.c

servers/slapd/back-ldap/init.c

servers/slapd/back-ldap/modify.c

servers/slapd/back-ldap/modrdn.c

servers/slapd/back-ldap/proto-ldap.h

servers/slapd/back-ldap/search.c

servers/slapd/back-meta/back-meta.h

tests/data/chain.out

tests/data/chainmod.out

tests/data/chainref.out

tests/data/test-chain1.ldif

tests/data/test-chain2.ldif

tests/scripts/defines.sh