MODPY_BIN_ADJ's perl snippt accepts multiple files so save a few execs.
To make it more robust, also append `--' to it such that ports cannot
(accidentially) pass options; I've checked the tree that no port does
this on purpose.
The only case where this could fail is with huge MODPY_ADJ_FILES but
that is not the case in our tree; ports where lots of shebangs are
fixed have their own construct around it, e.g. textproc/calibre which
uses the `find -exec ${MODPY_ADJ_FILES} {} +' idiom.
OK sthen
0ae5e05428