Further fixes to ezcod_5 5:10 location codes implementation
o Rename to ezcod_5, and parameterize both the number of parity
  symbols (default: 1), and location symbols (default: 9).
o Improve ezpwd::reed_solomon R-S API to separate supplied erasure
  positions from returned computed error positions; the supplied
  erasure vector is never modifed.
o Improve the R-S decoder to identify impossible error positions
  computed, and fail the decode.  This is an enhancement over the
  Linux kernel and Phil Karn's implementation, which allowed such
  error positions to be returned. TODO: fix exercise.H