random: Ingest extra fast entropy when !seeded

We periodically ingest entropy from pollable entropy sources, but only
8 bytes at a time and only occasionally enough to feed all of Fortuna's
pools once per second. This can result in Fortuna remaining unseeded
for a nontrivial amount of time when there is no entropy passed in from
the boot loader, even if RDRAND is available to quickly provide a large
amount of entropy.

Detect in random_sources_feed if we are not yet seeded, and increase the
amount of immediate entropy harvesting we perform, in order to "fill"
Fortuna's entropy pools and avoid having

random: randomdev_wait_until_seeded unblock wait

stall the boot process when entropy is available.

This speeds up the FreeBSD boot in the Firecracker VM by 2.3 seconds.

Approved by: csprng (delphij)
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D35802