In the lib, we should only #include "sigrok.h" or "sigrok-internal.h",
but not the (possibly installed and thus different/older versions) via
<sigrok.h> or <sigrok-internal.h>.
Frontends should of course use <sigrok.h> and <sigrok-internal.h>.
- Much more elaborate doxygen comments for the function and its
parameters, allowed input values, and so on.
- Make 'probelist' const, the function doesn't modify it.
- Check for invalid inputs such as NULL pointers.
- Use g_try_malloc() and g_free() for the output buffer. Document who
is expected to g_free() the buffer.