Make SCPI functions device independent, with separate serial backend.
Implement SCPI over USBTMC.
Add sr_scpi_read() operation for reading arbitrary data.
Port rigol-ds driver to use common SCPI functions.
Remove SR_INST_USBTMC which is no longer used.
Only scpi_serial.c functions need HAVE_LIBSERIALPORT, not scpi.c.
Make sr_scpi_send() take printf-style arguments.
Replace rigol_ds_send() function with sr_scpi_send().
Add sr_scpi_send_variadic() function.
rigol-ds: Eliminate fixed-size buffer in set_cfg.
rigol-ds: Support RS232 connection.

Probing tested OK over RS232 for DS1052E and DS1102D. Capture needs work.
scpi_serial: Flush buffers after opening port.
rigol-ds: Handle partial analog frame reads.

Reading a frame over the DS1xx2 RS232 connection now sometimes works,
but most of the time stalls part way through with g_poll showing the
fd as not ready.
Add implementation for SCPI over TCP.
rigol-ds: Support TCP connection.
scpi_tcp: Adjust to observed protocol for Rigol VS5000 series.
rigol-ds: Support VS5000 series devices.
scpi/serial: Use stubs for all SCPI functions.

Avoids some gcc warnings, since the SCPI prototypes don't exactly
match serial_*.
scpi/usbtmc: Prototype fixes.
rigol-ds: Minor error reporting fix.