Added support for personal overrides of the whole site-start.el and
specific startup files:

 1) If ~/.emacs.d/site-start.el exists, it is loaded instead of
    processing the rest of this file.  This way the user can turn
    off automatic loading of the system startup files completely.

 2) Startup *.el files are searched in ~/.emacs.d/site-start.d and
    /etc/emacs/site-start.d; personal files override the system
    startup files.  This way the user can turn off (or fix) the
    offending startup file without disabling this startup system
    altogether.

vsu@altlinux.ru, October 2002
