use BEGIN/END markers for gateway scripts

use BEGIN/END markers for gateway scripts

remove_gateway_scripts() for debian containers can easily
match user-created scripts since it's not strict enough,
which now that we always clean up gateways appears to be an
issue for some.

To deal with this we update this portion to also use
BEGIN/END markers like we did with other files. (Note that
this means we explicitly include the BEGIN/END comment lines
in the 'attribute' list while parsing sections.)

In order to not break existing container configurations we
now remove either a begin/end marked section in
remove_gateway_scripts(), or if none was found fall back to
a much stricter variant of the old matching algorithm which
only triggers if the gateway setup lines are complete and
unmodified.