xattr-helper: use upper caps for module constants
xattr-helper: add docstrings to more places
xattr-helper: use ValueError when the input is wrong

This is the standard exception for when the input is incorrect.
xattr-helper: standardize quoting style a bit
xattr-helper: refactor dump_xattrs to make it a bit more readable

Pull the common qoute chars out into a var and use % with format
strings rather than mixing + in between.
xattr-helper: clean up command line interface

This uses the standard argparse fields rather than open coding things
ourselves.  It also makes it a bit easier to test as you can now pass
paths on the command line in addition to stdin.