Improve sr_filter_probes error handling and docs.

Improve sr_filter_probes error handling and docs.

 - 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.