random: Make the entropy source registration interface more uniform

Most pure sources work under a "pull" model wherein a dedicated thread
polls the source at regular intervals (every 100ms). A couple of
sources, however, will instead call random_harvest_direct() to provide
entropy samples. Such sources were not calling random_source_register()
and thus weren't in the global random source list.

Modify "push" sources to use random_source_register() like other sources
do. Such sources omit an implementation of rs_read and are thus skipped
by the above-mentioned thread. This makes it easier to allow pure
sources to provide a min-entropy estimate in a uniform way.

Reviewed by: cem
MFC after: 2 weeks
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52229

(cherry picked from commit 8635f86977638eed966ec59cd319521fffb4df70)