Clean up init.el.  Move system-specific init files.

Remove custom-set-variables from init.el.  Move init-gnu-linux.el and
init-windows-nt.el into their own directories.  These directories might
contain other system-specific files in the future.
Move functions from system-specific init files.

Move functions whose impelementation is not os-specific into init.el.
Minor modifications to some functions.
Add new goals to TODO file.
Move package loading and some mode hooks to init.

Remove unnecessary package loading from system-specific init files.
Simplify useful modes for programming mode hooks and place in init.el.
Move keybindings from system-specific to general.

Move all keybindings in system-specific init files to general init file.
Move rest of common things from system-specific.

Move keybindings, enabled commands, themes, hooks and MISC from
system-specific init files to general init file.
Move section system-specific in init file.

Move section in order to define functions in the system-specific files
before they would be used in the general one.
Fix backup and auto-save file path.
Update TODO list.
Modify file-starting comments.

Replace name and remove date-line.  Make a comment clearer.