kern_nrandom, rnd drivers: update for new CSPRNG

kern_nrandom, rnd drivers: update for new CSPRNG

 * Identify each source of entropy with an identifier, so that
   for example the Fortuna CSPRNG can manage the pool index
   for each source.

 * Add a new sysctl, kern.rand_mode, which can be used to select
   which generator is used for /dev/random; valid values are:
    - csprng (to use only the Fortuna-based CSPRNG)
    - ibaa (to use only IBAA)
    - mixed (to XOR both csprng and ibaa in the output stream
   It defaults to "mixed".