random(4): Gather entropy from Pure sources

At initialization, hc_source_mask only includes non-Pure sources.

The patch changes source registration to enable the registered source in the
hc_source_mask bitmask. This mask governs which sources are harvested.

This patch also disallows userspace from disabling such sources.

PR: 222807
Submitted by: W. Dean Freeman <badfilemagic AT gmail.com>
Reviewed by: jmg (earlier version), delphij
Approved by: secteam (delphij)
Obtained from: HBSD 0054e3e170e083811acc9f3b637f8be8a86c03e7
Security: yes
Differential Revision: https://reviews.freebsd.org/D12611