input/vcd: Avoid truncation of shift to 32 bits.

(parse_contents): Cast constant 1 to uint64_t before shifting left.
Also fix whitespace errors.
input/vcd: Remove debug output from inner loops.

(parse_contents): Do not call sr_dbg() on every signal change.
This would be excessive even for sr_spew().
(read_until): Do not call ftell() just to be able to show some
number in a debug message later on.
output/vcd: Extend output stream to last sample.

(receive): Write a dummy timestamp after the last sample
to indicate the length of the sampled stream.
Fix building without librevisa.
rigol-ds: properly deal with dev_close() getting called multiple times
rigol-ds: properly end WAIT_TRIGGER event handling
rigol-ds: apply :KEY:LOCK DISABLE only to DS1K
libsigrok-internal.h: add helper macro to read 8 bits integer
matching the 16 and 32 bits one
libsigrok-internal.h: endian neutral helper macro to write 8/16/32 bits integer to unaligned memory
scpi: add a struct drv_context parameter to scpi_dev_inst_new()
scpi: add a generic scan API and implement it in usbtmc and serial transport

note that sr_usb_find_serial() is copied from the hameg-hmo driver
rigol-ds: use the new scpi scan API
hameg-hmo: use the new scpi scan API
scpi: add a libusb based implementation of usbtmc
usb: remove unused sr_usb_find_usbtmc()
scpi: Disable scpi_usbtmc in favor of scpi_usbtmc_libusb for now.

Having both in the list will "find" the same device twice.
scpi_usbtmc_libusb: Consistent libusb error handling.
scpi_visa: Add missing callback parameter.

(scpi_visa_dev_inst_new): Fix serious compiler warning due to
missing drvc parameter in prototype.
Update/extend some Doxygen comments.
gmc-mh-1x-2x: Cleanup, docs, minor fixes.