random: Have RANDOM_PURE_START be a cross-platform source

and one that will be with us in the long-term future.
(this helps reduce diffs in the future and for down-stream users
that trim entropy sources). Also, move deprecated (removed in 16.0)
sources to the bottom of the list to reduce changes to 15.x.

Approved by: re (cperciva)
Reviewed by: glebius
Obtained from: Juniper Networks
Differential Revision: https://reviews.freebsd.org/D53311
(cherry picked from commit 9f3886347c1750cf80a82314470fc7186088eb9a)
(cherry picked from commit e224f2c1b98ba065942c83d322cdb8e3987d0c81)