random(4): Translate a comment requirement into a compile-time invariant

In various places, random represents the set of sources as a 32-bit word
bitmask. It assumes all sources fit within this, i.e., the maximum valid
source number is 31.

There was a comment specifying this limitation, but we can actually refuse
to compile if our assumption is violated instead. We still have a few spare
random source slots, but sooner or later someone may need to convert the
masks used from raw 32-bit words to bitset(9) APIs.

This prevents some kinds of developer foot-shooting when adding new random
sources. No functional change.

Reviewed by: delphij, markm
Approved by: secteam (delphij)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D16982