udev: Add the Rigol DG4000 series to the udev rules
scpi_usbtmc: does not depend on libserialport
rigol-ds: does not depend on libserialport
Use PRIu64 for format in pretty-printer.

Thanks to Marcus Comstedt for the fix.
rigol-ds: prevent config_list() to return empty g_variant

This fixes the following glib message:
$ ./sigrok-cli -d rigol-ds --show
[...]
g_variant_builder_end: assertion '!GVSB(builder)->uniform_item_types || GVSB(builder)->prev_item_type != NULL || g_variant_type_is_definite (GVSB(builder)->type)' failed
rigol-ds: remove lonely break
scpi: properly check for HAVE_RPC (which is always defined)