We should use these (internal) functions in libsigrok exclusively from
now on, i.e. no more use of glib's g_debug() etc.
These functions are only for libsigrok, the frontends use whatever
logging mechanism is suitable there.
The struct entry 'extension' is not really a (filename) extension, but
rather a unique ID used for input or output formats, e.g. in the sigrok
CLI or GUI interface. Thus, rename it accordingly.