TextSegment: changed allocation semantics

A text_segment allocates memory in terms of chunks, this is now also
done, when created from a string, but this means that we can't rely on
strdup anymore, as it takes care of the allocation. Calling malloc
ourselves shouldn't be such an overhead as at least glibc's strdup
performs the exact same steps. Actually we should be spare a strlen call
now, so it should be more perfomant.