KASP: const specifier for entities serialization

src/dnssec/lib/dnssec/kasp.h

src/dnssec/lib/kasp/dir/dir.c

src/dnssec/lib/kasp/dir/keystore.c

src/dnssec/lib/kasp/dir/keystore.h

src/dnssec/lib/kasp/dir/policy.c

src/dnssec/lib/kasp/dir/policy.h

src/dnssec/lib/kasp/dir/zone.c

src/dnssec/lib/kasp/dir/zone.h

src/dnssec/lib/kasp/internal.h

src/dnssec/lib/kasp/kasp.c

src/dnssec/tests/kasp_store.c