source: Return error instead of implicitly reading from tty

For things like

    source $undefined

or
    source (nooutput)

it was quite annoying that it read from tty.

Instead we now require a "-" as the filename to read from the tty.

This does not apply to reading from stdin if it's redirected, so

    something | source

still works.

Fixes #2633.

tests/test1.in

@@ -213,6 +213,9 @@ echo 'echo "source argv {$argv}"' | source
 echo 'echo "source argv {$argv}"' | source -
 echo 'echo "source argv {$argv}"' | source - abc
 echo 'echo "source argv {$argv}"' | source - abc def
+# This hangs if it fails!
+source
+echo $status
 
 always_fails
 echo $status