use iswspace()/iswpunct() to find word delimiters

this inverts the configuration logic: you no longer provide a list of
delimiters -- all space and punctuation characters are considered
delimiters, unless listed in extrawordchars.

st.h

@@ -114,7 +114,7 @@ char *xstrdup(char *);
 extern char *utmp;
 extern char *stty_args;
 extern char *vtiden;
-extern wchar_t *worddelimiters;
+extern wchar_t *extrawordchars;
 extern int allowaltscreen;
 extern char *termname;
 extern unsigned int tabspaces;