parsergen: don't use 'frame' to pass args to shift() or receive from pop()

parsergen: don't use 'frame' to pass args to shift() or receive from pop()

'struct frame' holds a number of fields that shift()
ignores and pop() doesn't fill in.
So it is a bit confusing to see a frame passed in
and mostly ignored.

So just pass in the fields that are actually needed.
This fixes a bug where 'since_newline' was set wrongly when a newline
is shifted.

Signed-off-by: NeilBrown <neil@brown.name>