#2 bugfix for slip proxy

[Jason Turner]

patch for proxy_slip.c .

Two trivial bugfixes:

1/ The last two bytes of a packet were being skipped.
So any packet arriving on the slip proxy while the link
was down was corrupted. The invalid packets would still
be buffered, and then resent.
Probably the cause of the recorded bug "[459379] many
packets => link dies".

2/ Packets must be flushed (otherwise they can come
back to haunt you). See my notes posted a few days ago
about this in the discussion forum.

[Jason Turner]

Diff against proxy_slip.c v1.0