Added max packet size as arg,

parcel.cpp/h:
- Added -m as arg to pass max packet size. As is documented in the code,
the default size set (8400) was causing problems in certain cases, so it
was set to the default that UDT uses, which is 1500.  However, there may
be a speed change, so the ability to speed it up was added as a command
line arg.

udpipe_threads.cpp:
- Removed some debug output
- Made a few things safer in recvdata