libutil: Implement unix::fchmodatTryNoFollow, use in deletePath

[xokdvium]

Motivation

Using fchmodat after a fstatat in deletePath has a slight TOCTOU
window. We can plug it by using fchmodat (the libc wrapper with
AT_SYMLINK_NOFOLLOW), but it tries fchmodat2 and falls back to the
O_PATH trick while failing when procfs isn't mounted. We can do a bit
better than that and also cache whether syscalls are unsupported to
avoid the repeated context switching that glibc would impose.

Also tests the fallback path. It's only for kernels older than 6.6 and
when procfs isn't accessible that we fall back to the racy fchmodat
without AT_SYMLINK_NOFOLLOW.

What previously used to be:

openat(AT_FDCWD, "/tmp/store-race/nix/var/nix/builds", O_RDONLY) = 11
newfstatat(11, "nix-2704212-84654554", {st_mode=S_IFDIR|000, st_size=3, ...}, AT_SYMLINK_NOFOLLOW) = 0
fchmodat(11, "nix-2704212-84654554", 040700) = 0

Is now a TOCTOU-free sequence of syscalls:

openat(AT_FDCWD, "/tmp/store-race/nix/var/nix/builds", O_RDONLY) = 11
newfstatat(11, "nix-2704953-1733606057", {st_mode=S_IFDIR|000, st_size=3, ...}, AT_SYMLINK_NOFOLLOW) = 0
fchmodat2(11, "nix-2704953-1733606057", 040700, AT_SYMLINK_NOFOLLOW) = 0

Or if the fchmodat2 is not supported:

openat(11, "nix-2705443-3010460784", O_RDONLY|O_NOFOLLOW|O_CLOEXEC|O_PATH) = 12
fstat(12, {st_mode=S_IFDIR|000, st_size=3, ...}) = 0
chmod("/proc/self/fd/12", 040700)       = 0
openat(11, "nix-2705443-3010460784", O_RDONLY|O_NOFOLLOW|O_DIRECTORY) = 12

This prevents a potentially arbitrary chmod that follows symlinks,
though the race window is very small. Also in the case that fchmodat2
isn't supported we could instead open the /proc/self/fd/N path instead
of using openat, but that's pretty much equivalent. We only care
about ensuring that the thing we chmodded wasn't a symlink since
fchmodat follows symlinks and the support for AT_SYMLINK_NOFOLLOW
in libc for that is pretty spotty on Linux. E.g. glibc fails if the
AT_SYMLINK_NOFOLLOW is specified and procfs isn't available even on
regular files. The patch also includes a test that uses a user namespace
on Linux to test this exact scenario (though it's rather exotic).

Context

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