Allow passing the inline jq script before --

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

tests/shtest

@@ -579,4 +579,14 @@ if ( ! $msys && ! $mingw ) && locale -a > /dev/null; then
   fi
 fi
 
+# Allow passing the inline jq script before -- #2919
+if ! r=$($JQ --args -rn -- '$ARGS.positional[0]' bar) || [ "$r" != bar ]; then
+    echo "passing the inline script after -- didn't work"
+    exit 1
+fi
+if ! r=$($JQ --args -rn 1 -- '$ARGS.positional[0]' bar) || [ "$r" != 1 ]; then
+    echo "passing the inline script before -- didn't work"
+    exit 1
+fi
+
 exit 0