The file names are separated with newlines in the primary and with spaces in
the clipboard.

Thanks ploth for coming up with this:
	https://github.com/muennich/sxiv/issues/247#issuecomment-321316306

Replaces optional dependency on xsel(1) to xclip(1) as it supports this.

Fixes issue #247

Misbehaving command lines in the key-handler that rewrite the given files and
thus replace symbolic links instead of their targets shall call realpath(1).

Fixes issue #205 and reverts commit 92e3b578.

The default prefix key is C-x, and can be changed in config.def.h. The
first key pressed after the prefix key will be passed the external key
handler, unless the key is Escape, which is used to cancel the prefix.

Gets called on all unset key mappings. Arguments are: key combo and
current file. Thanks to Francesco Orsenigo (xarvh) for the idea.