.emacs: Some trivial changes.

- Recenter after jumping to next or previous diff in diff-mode.
- Use c++-mode in *.c and *.h files.
- Find *.[ch], *.cc and *.cpp files for Grep.
- Changed some settings for compilation.
- Disabled recursive use of minibuffer.
- Enabled global-hl-line.
- Use ssh as rlogin-program.
- Changed letter to be used for fill-column in ruler-mode.
- Not use GUI dialog to select file.