string: always call strcmp() in nstrcmp()

Some people seem to depend on the unstandardized
behavior of some (notably glibc) implementations
of strcmp() where the arithemtic difference
between s1 and s2 is returned, not just any
positive or negative number.  This is explicitly
undocumented in libneo as well, but still doing
it won't hurt either.

src/string/nstrcmp.c

@@ -22,13 +22,13 @@ int nstrcmp(const string *s1, const string *s2, error *err)
 
 	int ret;
 
-	if (nlen(s1) > nlen(s2))
-		ret = 1;
-	else if (nlen(s1) < nlen(s2))
-		ret = -1;
+	usize maxbytes;
+	if (s1->_capacity > s2->_capacity)
+		maxbytes = s2->_capacity;
 	else
-		ret = strcmp(s1->_data, s2->_data);
+		maxbytes = s1->_capacity;
 
+	ret = strncmp(s1->_data, s2->_data, maxbytes);
 	neat(err);
 	return ret;
 }