error.c: Add SR_ERR_DEV_CLOSED.
output/ols: Use new API

This now properly supports samplerate changes coming in from
SR_DF_META packets, and also fixes bug 99.
gnuplot: fix for big-endian architectures.
vcd output: fix for big-endian architectures.
vcd input: Use a GSList rather than fixed size array of probes.
text: Use a GSList of enabled probe names, not an array.
chronovu-l8: remove unused array of probe names.
csv: Remove unnecessary array of probe names.
gnuplot: Remove unnecessary array of probe names.
gnuplot: Eliminate fixed-size header buffer based on max probes.
vcd output: Eliminate unnecessary array of probe names.
vcd output: Replace fixed length array of indices with GArray.
Remove SR_MAX_NUM_PROBES, which is now no longer used.
Fix output API receive() function

This changes the new output API's recv() to receive(), and has it
return an integer status code. The output of the function, if any,
is stored in a new parameter as a pointer to a newly allocated GString.

All output modules using this API have been adjusted.
Explicitly initialize all struct parameters on init
Don't automatically clear known instances from USB drivers on scan

This invalidates previously returned sr_dev_inst pointers, which a
frontend may be holding. It's the frontend's responsibility to clear
the list of instances a driver keeps track of by calling

	sr_dev_clear(driver);

if it wants a completely new scan done.
unittests: Disable a broken test for now.
tondaj-sl-814: Use sr_dev_inst to store connection handle.
NEWS: Add most important items since last release.
Don't try to close devices when removing them from the session

That's the frontend's job.