i2c: Add read ability

This adds the ability to read from I2C registers.
packet: Add buffer drain command

This command lets the receiver know that the buffer is (or will be)
draining.
main: Use new buffer_drain packet

When we start to drain the buffer, emit a buffer_drain packet.  When
we're done, emit a buffer_drain_done packet.
fpga: Support toggling READ line, check for duplicates

As a sanity check, monitor the line for duplicates.

Also, the READ line now responds to toggles rather than simply
rising edges.
parse: Remove extra line from help screen

It did nothing.
main: Add hello command

This adds a "greeting" command that can be used to identify a stream.
net: Switch from UDP to TCP

We were losing packets when using UDP.