Add help message

Change-Id: Id9c5dfdee00c8b9baa21c7345f7218e982bfd434
Improve command line handling, don't read stdin

Reading stdin is confusing as it means running the script without any
arguments and with no input redirection just seems to do
nothing. (OTOH, who would use this script except seasoned hackers who
know how command-line tools typically work... oh well.)

Change-Id: I00b4f70b07b6515b52a22b4ec4e048cc84c1dc83