random: Avoid magic numbers

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.

Reviewed by: cem
MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46693

(cherry picked from commit 32fce09268ddd97efb4412529ba57293554c5985)