Start of code base layout restructuring.
Rename libbackend to libsigrok.
Initial Value Change Dump (VCD) output support.
More consistent spelling of "samplerate".
Allow output_format.init() to return errors.
Start unification of libsigrok return codes.

We have SIGROK_OK for functions calls where no errors occured. All
error code names start with SIGROK_ERR and are globally unique,
negative values.

The value SIGROK_ERR is a generic/unspecified error code, all others,
such as SIGROK_ERR_MALLOC, refer to a specific error condition.

This commit renames the old SIGROK_NOK etc.
libsigrokdecode: Add initial return code list.
Gnuplot output format support.
VCD output: Handle disabled probes correctly.
Factor out common sigrok_samplerate_string().
fix off-by-one error
better buffer size calculation
use strdup() instead of g_strdup()
various fixes, basic acquisition works
switch to SIGROK_ERR
move posix-specific serial port comms to serial.c
Coding style fixes, aided by 'indent'.

The following shell alias can be used:

  alias INDENT='indent -npro -kr -i8 -ts8 -sob -l80 -ss -ncs'
Simplify code by reducing nesting level.
sl_open_device(): Refactoring to reduce nesting.
ezusb.c: Coding style fixes.