fsx: seed more randomly with the -S0 option

When using -S0, seed the PRNG with the current time in nanoseconds, not
seconds, so consecutive runs don't accidentally use the same seed.

Also, rename some variables for clarity.

Reviewed by: ngie
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20078