Add support for cachestat() syscall

[oandrew]

Linux 6.5 added new cachestat() syscall to obtain page cache statistics for a given fd. It is much faster than mincore(), especially on big files. Update vmtouch to use cachestat() when it's available and none of the -t, -e, -v flags are passed. Additionally, cachestat() reports correct numbers even without write permissions on files. (https://lwn.net/Articles/778437/)

On my machine, running vmtouch on directory with:

• Linux kernel checkout (96962 files, 7Gb total)

Before: 1.2s 
After:  0.6s

• Large media files (417 files, 139Gb total)

Before: 800ms
After:   13ms