.emacs: Some trivial changes.

- Disabled electric-indent-mode, but bound C-m to newline-and-indent.
- Use whitespace-mode only in buffers bound to a file.
- Use my-browse-url function only when DISPLAY is not w32.
- Removed advice for diff-func-next and diff-func-prev.
- Enabled flyspell-prog-mode in c++-mode.
- Use nice in make-gtags advice.  And in this advice, previously created
  buffer and process are removed before create new ones.
- Enabled gdb-many-windows.
- Display *Help* buffer in same window.
- Added SKK rule to wide char for round brackets.
.aspell.conf: Changed default language to en_US.