random(4): Match enabled sources mask to build options

r287023 and r334450 added build option mechanisms to permanently disable
spammy and/or low quality entropy sources.

Follow-up those changes by updating the 'enabled' sources mask to match.
When sources are compile-time disabled, represent them as disabled in the
source mask, and prevent users from modifying that, like pure sources.
(Modifying the mask bit would have no effect, but users might think it did
if it was not prevented.)

Mostly a cosmetic change.

Reviewed by: markm
Approved by: secteam (gordon)
X-MFC-With: 334450
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17252