Clean up sr_session_load().

This cleans up some warnings generated by clang's static analyzer.
The function now also returns SR_ERR to signify the specified filename
does not point to a valid session file.

Other SR_ERR_* returns indicate a session file was found, but loading
failed.
sr_parse_sizestring: Deal with invalid strings better.

This cleans up a warning generated by clang's static analyzer.
input: Avoid NULL dereference.

This cleans up a warning generated by clang's static analyzer.
input/binary: Code cleanup.

This cleans up a warning generated by clang's static analyzer.
input/chronovu_la8: Code cleanup.

This cleans up a warning generated by clang's static analyzer.
input/csv: Code cleanup.

This cleans up a warning generated by clang's static analyzer.
scpi/vxc: Avoid NULL dereference.

This cleans up a warning generated by clang's static analyzer.
agilent-dmm: Code cleanup.

This cleans up a warning generated by clang's static analyzer.
asix-sigma: Trigger code cleanup.

This cleans up a warning generated by clang's static analyzer.
chronovu-la8: Code cleanup.

This cleans up a warning generated by clang's static analyzer.
fluke-dmm: Code cleanup.

This cleans up a warning generated by clang's static analyzer.
demo: Code cleanup.

This cleans up a warning generated by clang's static analyzer.
hantek-dso: Code cleanup.

This cleans up a warning generated by clang's static analyzer.
kecheng-kc-330b: Fix missing time/frequency weighting.
Fix similar broken error handling on several serial calls.
Avoid serial_write_blocking() warnings.
Add a timeout parameter to blocking serial calls.

Set this new parameter to 0 (no timeout) at every call site. This is
consistent with previous behaviour, so cannot cause any regressions.

Waiting forever for a serial operation is clearly always wrong. Without
specific knowledge of each device and driver however, I can't choose
appropriate timeouts for each call. The maintainers of these drivers
will need to do so, and also add appropriate handling of timeouts.

When this commit is merged, a bug should be entered for each driver
that is touched by it.
Add some Agilent USBTMC PIDs.
Add a configurable read timeout to blocking SCPI reads, default 1s.
unit tests: Fix build due to recent changes.