interactive/psk: Add 500 and 1000 modes

We now also print a warning and return false if a user enters a bad
value.

Signed-off-by: Joel Stanley <joel@jms.id.au>
interactive: clean up bpsk code

Signed-off-by: Joel Stanley <joel@jms.id.au>
interactive: allow user to select BPSK rate

UI looks like this:

  Select BPSK baud rate: (default is 31)

          31

          63

          125

          250

          500

          1000

  > 63

  Starting BPSK Terminal.
  Press ` to exit.

Signed-off-by: Joel Stanley <joel@jms.id.au>