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.