random(4): Add is_random_seeded(9) KPI

The imagined use is for early boot consumers of random to be able to make
decisions based on whether random is available yet or not. One such
consumer seems to be __stack_chk_init(), which runs immediately after random
is initialized. A follow-up patch will attempt to address that.

Reported by: many
Reviewed by: delphij (except man page)
Approved by: secteam(delphij)
Differential Revision: https://reviews.freebsd.org/D19926