Allow reading a list of file names from standard input

This PR adds the ability to read a list of file names from standard input if the only filename specified on the command line is `-`.  This allows lists of files longer than the maximum command line length to be passed to duperemove and makes it easier to use a list of files piped from another command.

I've also included another commit which fixes a couple of small typos in the `duperemove` man page which I noticed while I was updating it.