Remove disable_reuseport config option

This was leftover from an earlier iteration of the SO_REUSEPORT
  code, but in the current implementation it's pointless, as
  SO_REUSEPORT is used iff it was runtime-detected *and* the user
  has configured more than one thread-per-address.
In the case that some platform has SO_REUSEPORT bugs/behaviors that
  gdnsd didn't anticipate, one can always leave threads at the
  default value of 1 to avoid using it, with the same effect as
  this option.