Add support for cachestat syscall as mincore alternative

[boomskats]

This adds support for the cachestat syscall introduced in kernel 6.5 by @nhatsmrt as an alternative to mmap/mincore. I've tried to make sure that the output is fully backward-compatible and users have to explicitly opt-in to the mechanism. The only output that's changed is the -json output where I've added the newly available stats if and only -cachestat is chosen.

There's some good background on the new syscall here. I guess the headline is this:

Using cachestat
real -- Median: 33.377s, Average: 33.475s, Standard Deviation: 0.3602
user -- Median: 4.08s, Average: 4.1078s, Standard Deviation: 0.0742
sys -- Median: 28.823s, Average: 28.8866s, Standard Deviation: 0.2689

Using mincore:
real -- Median: 102.352s, Average: 102.3442s, Standard Deviation: 0.2059
user -- Median: 10.149s, Average: 10.1482s, Standard Deviation: 0.0162
sys -- Median: 91.186s, Average: 91.2084s, Standard Deviation: 0.2046

I also ran both syscalls on a 2TB sparse file:

Using cachestat:
real 0m0.009s
user 0m0.000s
sys 0m0.009s

Using mincore:
real 0m37.510s
user 0m2.934s
sys 0m34.558s

I'm going to run some more tests over the next few days, but it looks like 15GB ish is the sweet spot where the new syscall starts to make a difference.