fx2lafw: fix possible use of uninitialized variable (gcc-4.9 warning fix)

hardware/fx2lafw/protocol.c: In function 'fx2lafw_command_start_acquisition':
hardware/fx2lafw/protocol.c:113:7: warning: 'cmd.flags' may be used uninitialized in this function [-Wmaybe-uninitialized]
   (cmd.flags & CMD_START_FLAGS_CLK_48MHZ) ? "48" : "30");
       ^
scpi_usbtmc_libusb: disable bulk_out clear_halt for Rigol DS1000

fix hangup issue with Rigol firmware version 00.02.02.02.00
scpi_usbtmc_libusb: disable all clear_halt for Rigol DS1000
rigol-ds: fix long memory depth setting on DS1000
configure.ac: Clearly mark required and optional libs.
chronovu-la: Show errors other than "device not found".

This can be useful for the user to see, e.g. for permission related
issues (device is there, but cannot be accessed).
sr_dev_inst_new(): Use NULL (not "") if version unknown/nonexisting.

Unify the whole code-base to use NULL there, not a mix of "" and NULL.
.gitignore: Add test-suite.log.
output/gnuplot: Use new output API.
output/vcd: Use the logic stream's unitsize.

Also code cleanup and some memory leaks fixed.
output/csv: Use the logic stream's unitsize.

This also fixes the channel index, instead of counting on the logic
data being arranged according to enabled channels only.
output/chronovu_la8: Use new output API.

This somewhat naively copies whatever it gets into the output, regardless
of how many channels are in there, or which ones are enabled. Not sure
what the best way to deal with that is, but for now you have to feed it
a channel setup the Chronovu software can read.
output/binary: Use new output API.
output/bits: Rewrite for new output API.

This also fixes the performance problems in the old module.
output/hex: Rewrite for new output API.
output/ascii: Rewrite for new output API.
Remove obsolete output API.
Removed obsolete sr_filter_channels().
Removed ALSA driver.

This driver has been unmaintained for years, and was never good code
to begin with. It's also questionable whether it was ever useful,
particularly with the demo driver now supporting various analog
signalling.
output: Introduce output module API wrappers.