reimplemented SH_Text as single linked list of strings

The (intern) implementation of SH_Text was changed from an array of
char, to a single linked list of arrays of char. This allows an easier
implementation of (further) text manipulation.
The API hasn't changed much, but SH_Text_join can't yield an error
anymore, so it now doesn't support passing an error and returns nothing.

The source has been adapted to splint, which still tells about some
errors, but they are all checked to be false-positives.