Modify xclip selection parameter

On my machine (Ubuntu 12.04), Clipboard would only
affect the local terminal rather the global clipboard.
This appears to be an issue with calling xclip with
"-selection primary".  Using "-selection clipboard" seems
to work.  This needs more investigation.
Add -filename filter

Add an option to the clipfilter program that filters out
invalid file name characters.  The list of allowed characters
should probably be reduced, but this works so far on Linux.