Reintroduce cpu_rng_early_sample(), but this time with embedded detection
for RDRAND/RDSEED, because TSC is not very strong.

Simplify Intel RDRAND/RDSEED and VIA C3 RNG API.

Push it all into MD x86 code to keep it simpler, until we have other
examples on other CPUs. Simplify RDSEED-to-RDRAND fallback.
Eliminate cpu_early

Simplify Intel RDRAND/RDSEED and VIA C3 RNG API.

Push it all into MD x86 code to keep it simpler, until we have other
examples on other CPUs. Simplify RDSEED-to-RDRAND fallback.
Eliminate cpu_earlyrng in favour of just using entropy_extract, which
is available early now.

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More ASLR. Randomize the location of the direct map at boot time on amd64.
This doesn't need "options KASLR" and works on GENERIC. Will soon be
enabled by default.

The location of the areas is abstr

More ASLR. Randomize the location of the direct map at boot time on amd64.
This doesn't need "options KASLR" and works on GENERIC. Will soon be
enabled by default.

The location of the areas is abstracted in a slotspace structure. Ideally
we should always use this structure when touching the L4 slots, instead of
the current cocktail of global variables and constants.

machdep initializes the structure with the default values, and we then
randomize its dmap entry. Ideally machdep should randomize everything at
once, but in the case of the direct map its size is determined a little
later in the boot procedure, so we're forced to randomize its location
later too.

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Add cpu_rng, a framework for simple on-CPU random number generators.