some versions of perl failed to run packet.pl because the 'last'
keyword can't be used outside of a loop block. this commit changes
the packet dumping code to a function so we can use 'return' instead.
the tcp system test uses the 'packet.pl' test tool to send a packet
thousands of times. this took a long time because the tool was waiting
for replies and parsing them; however, for that particular test the
replies aren't relevant.
this commit uses non-blocking sockets and moves the reply parsing
outside the send loop, which speeds the system test up substantially.
For some tests, we need to send big data streams (for TCP) or repeated
packets (for UDP), this commits adds `-r` option to packet.pl that sends
the same input <repeats> times using the specified protocol.
