roots: use openat2 instead of openat when available

roots: use openat2 instead of openat when available

This increases resistance to symlink and mount attacks.

Previously, bees could follow a symlink or a mount point in a directory
component of a subvol or file name.  Once the file is opened, the open
file descriptor would be checked to see if its subvol and inode matches
the expected file in the target filesystem.  Files that fail to match
would be immediately closed.

With openat2 resolve flags, symlinks and mount points terminate path
resolution in the kernel.  Paths that lead through symlinks or onto
mount points cannot be opened at all.

Fall back to openat() if openat2() returns ENOSYS, so bees will still
run on kernels before v5.6.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>