asix-sigma: add support for "probe names" scan option

asix-sigma: add support for "probe names" scan option

Add support for the SR_CONF_PROBE_NAMES scan option. Optionally assign
user specified channel names instead of the driver's builtin names.

This commit is a little more involved because channel names are also
referenced as external clock source. The lack of 'const' decoration in
the glib API and the presence of 'const' in the std_*() routines require
some intermediate storage and casts and duplicate length determination.
This is considered more desirable than a redundant length field in devc.