zfsloader: messy output on Dell iDrac serial console

[Julian Elischer]

running on *some* Dell servers the serial console looks messed up at 
the time of the FreeBSD menu.
it looks like it's dropping (or adding? but that's less likely) 
characters.

  |  5. [K]ernel: kernel (1 of 2)           |  .- .                    -.
  |  6. Configure Boot [O]ptions...         |2  --| y/       -.       
-/`   -o/
  |137. Select Boot [E]nvironment...        | `:`                  
`:`         ::/sy++:.
  |                                         H|     .-- `--.     --  
/6H`:                          :`
  |                                         | .---.....----.

IS there an RTS/CTS input we should be attending to?

I will add that some of the other modules (e.g. the mfi raid setup 
section) also get affected,
so it's not just a bug in our code.