Improve scrollback file handling

- Properly use filesystem encoding
- Validate utf8 when loading (hopefully fixing crashes)
- Use Gio
- Handle Windows line endings
- Remove dead code
- Fix respecting max length of scrollback files

src/common/hexchat.c

@@ -447,7 +447,6 @@ session_new (server *serv, char *from, int type, int focus)
 
 	sess->server = serv;
 	sess->logfd = -1;
-	sess->scrollfd = -1;
 	sess->type = type;
 
 	sess->alert_beep = SET_DEFAULT;