Recognize xsel as valid X11 clipboard app.
Better detection/configuration of X11 clipboard app.

On X11 systems, the new user-configurable variable
`g:fakeclip_preferred_x_clip_app` can now be set to either `'xsel'` or
`'xclip'` by the user in `~/.vimrc`.  If left undefined, the script will
probe the availability of `xsel` and `xclip`, preferring the latter if
possible.

With `g:fakeclip_preferred_x_clip_app`, the user doesn't have to
manually specify the full command options for reading/writing the
clibpoard/primary.  The user can still use the
`g:fakeclip_{read/write}_{clipboard/primary}_command` variables, which
take precedence.  On non-X11 systems, the
`g:fakeclip_{read/write}_{clipboard/primary}_command` variables can
still be used.
Merge branch 'smarter-detection-of-x11-backends'