random: Allow pure entropy sources to provide a min-entropy estimate

The current implementation of the NIST health tests assumes a
min-entropy estimate of one bit per sample, which is quite conservative.
For so-called "pure" sources (e.g., virtio-random, TPM) it might be nice
to support larger estimates so that the tests catch failed devices more
quickly.

Thus:

• let each pure random source provide an estimate, so that downstreams or driver implementors can override defaults if they want to;
• increase the default estimate for pure sources;
• for pure sources initialize the state machine at source registration time.

Reviewed by: cem
MFC after: 2 weeks
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52232

(cherry picked from commit f865264f6a5eba4025c0f6284a48f383717fd74e)