jq previously only allowed passing the inline script before -- (as if
they were options) even though one would expect the inline script to be
a positional argument.
Since jq previously also refused to run with a usage error if the script
was passed after -- (It was not assuming . as script as it does when
no arguments are passed), and positional arguments are allowed before --
and even before other options, it should not be a breaking change to
change that weird behaviour, and allow the script to appear after --.
It also simplifies the option parsing code a bunch.
Fixes#2918