parser: drain all leftover buffers between blocks

[otavio]

A comment or def block peeks the line that terminates it and returns it as a
leftover buffer for the main loop to re-process. When that leftover is itself a
comment/def block, re-processing produces a second leftover — but the loop
only drained one level (if nextbuf:), so the second leftover was glued onto
the next raw line instead.

In practice this misparsed an addhandler line followed by its
python ...() {} event handler when the handler was preceded by two or more
def helper blocks: the handler merged with the addhandler line and its body
spilled out as loose Items instead of a single Function.

Drain the leftover buffer fully (while nextbuf:). Adds a regression test
covering a handler that follows two def helper blocks; the full suite (178
tests) passes.

Note: with this change the line = nextbuf + line glue in the same loop is now
always a no-op (the buffer is fully drained before the next raw line is read).
I left it in place to keep this fix minimal, but it could be removed in a
follow-up cleanup if you prefer.

[priv-kweihmann]

Thx for this fix - and yeah feel free to do cleanup changes at any time