Add missing `-help` function
-help
is mentioned in the manpages, but actually running xprop -help
results in xprop exiting with status 1 as it doesn't recognise the option.
This has been the source of major confusion here:
https://github.com/franciscolourenco/done/issues/77#issuecomment-612404448
Signed-off-by: Jason Nader jason.nader@protonmail.com