output/csv: Drop last separators as needed.
rigol-ds: Fix a couple of compile warnings.
sr file format: Bump version to 2.

The *.sr (libsigrok session) file format has changed since the last
libsigrok release. Frontends using older libsigrok versions will not
be able to read *.sr files created by frontends using the new file format.

Thus, bump the version number of the file format to 2.

Current libsigrok will read both version 1 and version 2 files
correctly, and always write version 2 files.