A replacement for g_strsplit_set(). While g_strsplit_set()
does (n+1) malloc and n slice allocations, this needs
roughtly (O(log(n))) mallocs.

Another difference from g_strsplit_set() is that this function
treats multiple delimiters as one (and thus never returns empty
words). While I can see that sometimes you may want to keep empty
words (like parsing a CSV file and preserve empty cells), we usually
use this function for splitting user input. In such case, we want
to treat multiple delimiters as one.