getline: Kill GLS_MORE

There's no point in calling memchr() on each read just to determine if
the _next_ read might block.

Instead, let's provide a method to get that information from a struct
getline: getline_hasmore().
getline_clearbuf: clear the input buffer

This method is not needed for crashgui, but it's part of a general
cleanup and extension of this "mini-library".