Distinguish clipboard from primary selection on X11 systems.

On X11 systems, there are actually three distinct "clipboards", or X
selections, namely the `PRIMARY`, `SECONDARY` and `CLIPBOARD`
selections.  To emulate Vim's behaviour under X11 (see vim documentation
`|quote+|`), we now use the `+` register for accessing the
`CLIPBOARD` selection, and the `*` register for the `PRIMARY` selection.

On non-X11 systems, the distinction is probably moot.  Therefore, we try
to do nothing new, and both the `+` and the `*` registers will access
the same clipboard.
Simplify the generation of default keymappings.
Load once and save/restore cpo.
Add user-defineable options for reading the clipboard(s).

Following 62965c8, two user-defineable options are added:

*  `g:fakeclip_read_clipboard_command`
*  `g:fakeclip_read_primary_command`

They can be specified in `~/.vimrc`, overriding the default.