random: Avoid magic numbers

[cperciva]

Summary

Move RANDOM_FORTUNA_{NPOOLS,DEFPOOLSIZE} from fortuna.c to fortuna.h
and use RANDOM_FORTUNA_DEFPOOLSIZE in random_harvestq.c rather than
having a magic (albeit explained in a comment) number. The NPOOLS
value will be used in a later commit.

Sponsored by: Amazon

[cem]

LGTM.

[cem]

sys/dev/random/fortuna.c

86–89

Note (for the other change) that pool size is hypothetically tunable.