configure.ac: Disable atten-pps3xxx driver if libserialport is not found.
import VXI RPC Language description

along with the corresponding rpcgen generated source code files
scpi: add VXI transport support
scpi: factorize dev_inst_new calls out of individual drivers
scpi: make the scpi_dev_inst_new more generic
scpi_tcp: split into scpi_tcp_raw and scpi_tcp_rigol

The current implementation is renamed to tcp-rigol as it seems to be
a Rigol proprietary protocol used only on Rigol VS5000 series.

A new tcp-raw implementation is introduced which simply carries raw SCPI
commands over TCP. It is probably a much more common protocol and it is
at least available on Rigol DS2000 series on port 5555.
scpi_vxi: ensure the vxi link was properly opened before closing it
demo: Split supported device options by probe group.
demo: Properly handle logic vs. analog when setting the pattern.
atten-pps3xxx: Fix output channel mode get.
atten-pps3xxx: Fix options reporting.
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)
cem-dt-885x: Don't use char as signed type.
rigol-ds: Use correct digital channel numbers when fetching config.