Fortuna: Add failpoints to simulate initial seeding conditions

[cem]

Summary

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).

The goal is to be able to reproduce initial seeding transition problems, e.g.,
Lev's recent CURRENT report about failing newfs arc4random(3) usage.

Test Plan

# Block:
$ sudo sysctl debug.fail_point.random_fortuna_pre_read='return(1)'
debug.fail_point.random_fortuna_pre_read: off -> return(1)
$ sudo sysctl debug.fail_point.random_fortuna_seeded='return(1)'
debug.fail_point.random_fortuna_seeded: off -> return(1)

# Start test operation that uses devrandom and blocks on seeded status:
$ [truss] ./blocked_random_poc
...

# Unblock:
$ sudo sysctl debug.fail_point.random_fortuna_pre_read='off'
debug.fail_point.random_fortuna_pre_read: return(1) -> off
$ sudo sysctl debug.fail_point.random_fortuna_seeded=off
debug.fail_point.random_fortuna_seeded: return(1) -> off

[markm]

sys/dev/random/fortuna.c

398

Please put this inside the #ifdef _KERNEL

466

Again, inside #ifdef _KERNEL, please

[cem]

sys/dev/random/fortuna.c

398

Will do.

[cem]

ifdef _KERNEL failpoint sites