tests: Fix a few warnings.

Fix a bunch of "no previous prototype for ..." warnings exposed
by -Wmissing-prototypes.
sysclk-lwla: Implement SR_CONF_LIMIT_MSEC.

Allow the acquisition to be constrained by time in addition to
a sample count limit.  Since the LWLA protocol actually provides
only a duration natively, implement the sample count limit on top
of the new duration limit.

With this change, limiting an acquisition in external clock mode
should finally work properly.
sysclk-lwla: Streamline packet output logic.

(process_sample_data): When expanding run-length samples into
session packets, calculate the number of samples to write in
advance while honoring all constraints.  This is cleaner than
checking constraints within the expansion loop.  Also, the new
logic always fills up packets exactly to whatever limit applies
first, thereby removing the need for truncation after the fact.
sysclk-lwla: Improve message log output.

Report settings at acquisition start as informational messages.
Print a message when the the trigger condition has been met.
Demote some other messages from information to debug, and use
the %zu format for printing size_t values.
Remove SR_CONF_MAX_UNCOMPRESSED_SAMPLES again.

The maximum sample size that can be set on a device is now published
by sr_config_list(SR_CONF_LIMIT_SAMPLES). This returns a tuple of
uint64_t representing minimum and maximum number of samples.
rigol-ds: Update which channels are enabled after making changes.
rigol-ds: Check SCPI read is complete after reading expected block length.
rigol-ds: Correct digital waveform block sizes.
rigol-ds: DS1000 series actually use IEEE488.2 data block format.
scpi: Strip trailing newlines in sr_scpi_get_string().
rigol-ds: Use standard sr_scpi_get functions, remove internal versions.
scpi: Remove redundant newline removal in sr_scpi_get_hw_id().
rigol-ds: Use *OPC? command rather than delay to await completion.
rigol-ds: Cleanup and fix check for short data blocks.
rigol-ds: Send FRAME_BEGIN and FRAME_END per frame, not per channel.
rigol-ds: fix set_cfg() calls with float parameters to avoid locale issues
rigol-ds: add more valid timebases for DS2302
rigol-ds: use appropriately defined NUM_VDIV constant
rigol-ds: add support for more keys in config_get()
hwdriver: Introduce sr_config_commit() API call.

(sr_dev_driver.config_probe_set): New optional callback enabling
drivers to be notified upon changes to probe settings.
(sr_dev_probe_enable, sr_dev_trigger_set): Invoke new driver
callback on changes.
(sr_dev_driver.config_commit): New optional callback allowing
drivers to defer application of configuration settings until
an explicit call to config_commit().
(sr_config_commit): New public wrapper function.