Backport fixes from FreeBSD-12 to help the random(4) device thread

Backport fixes from FreeBSD-12 to help the random(4) device thread
not overwhelm the OS:

a) Use the correct symbolic constant when calculating 10'ths of a
second. This means that expensive reseeds happen at ony 1/10 Hz,
not some kHz.

b) Rate limit internal high-rate harveting efforts. This stops the
harvesting thread from total overkilling the high-grade entropy-
gathering work, while still being very conservatively safe.

PR:		230808
Reported by:	danilo,eugen
Tested by:	eugen
Approved by:	so (blanket permission granted as I am the authour of this code)
Relnotes:	Yes