kdig: allow optional SOA serial with NOTIFY query

doc/man/kdig.1in

doc/man_kdig.rst

src/utils/common/params.c

src/utils/common/params.h

src/utils/kdig/kdig_exec.c

src/utils/kdig/kdig_params.c

src/utils/kdig/kdig_params.h

src/utils/khost/khost_params.c