Add copyright notices

servers/slapd/back-shell/abandon.c

servers/slapd/back-shell/add.c

servers/slapd/back-shell/bind.c

servers/slapd/back-shell/compare.c

servers/slapd/back-shell/config.c

servers/slapd/back-shell/delete.c

servers/slapd/back-shell/external.h

servers/slapd/back-shell/fork.c

servers/slapd/back-shell/init.c

servers/slapd/back-shell/modify.c

servers/slapd/back-shell/modrdn.c

servers/slapd/back-shell/result.c

servers/slapd/back-shell/search.c

servers/slapd/back-shell/shell.h

servers/slapd/back-shell/unbind.c