Fortuna: Add failpoints to simulate initial seeding conditions

Set debug.fail_point.random_fortuna_pre_read=return(1) and
debug.fail_point.random_fortuna_seeded=return(1) to return to unseeded
status (sort of). See the Differential URL for more detail.

The goal is to reproduce e.g. Lev's recent CURRENT report[1] about failing
newfs arc4random(3) usage (fixed in r338542).

No functional change when failpoints are not set.

[1]: https://lists.freebsd.org/pipermail/freebsd-current/2018-September/071067.html

Reported by: lev
Reviewed by: delphij, markm
Approved by: secteam (delphij)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17047