mountpoint: add --show option to print mountpoint path

mountpoint: add --show option to print mountpoint path

Add a new --show option that prints the actual mountpoint path for
a given directory or file. This is useful for:
- Resolving any path to its containing mountpoint
- Finding the canonical mountpoint path when symlinks are involved
- Determining the mountpoint from paths within filesystems

The option requires kernel support for statmount(2) (Linux 6.8+).
On older kernels without statmount support, the option fails with
an error message, as the /proc/self/mountinfo fallback cannot
resolve arbitrary paths to their containing mountpoint.

Example usage:
  $ mountpoint --show /
  /
  $ mountpoint --show /home/user/file.txt
  /home

The --show option always returns EXIT_SUCCESS (0) when it successfully
finds the mountpoint, regardless of whether the given path itself is
a mountpoint or not.

Addresses: https://github.com/util-linux/util-linux/issues/3806
Signed-off-by: Karel Zak <kzak@redhat.com>