Import latest changes from devel include substrings indexing support

doc/devel/asn-layman.txt

doc/devel/guidelines

libraries/liblutil/signal.c

servers/slapd/back-ldbm/attr.c

servers/slapd/back-ldbm/back-ldbm.h

servers/slapd/back-ldbm/filterindex.c

servers/slapd/back-ldbm/index.c

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

servers/slapd/index.c

servers/slapd/proto-slap.h

servers/slapd/schema/core.schema

servers/slapd/schema/openldap.schema

servers/slapd/schema_init.c

servers/slapd/slap.h

tests/data/slapd.conf