mountpoint: use statmount() syscall on modern kernels

mountpoint: use statmount() syscall on modern kernels

Improve mountpoint(1) to use the modern statmount() system call
(available since Linux 6.8) instead of parsing /proc/self/mountinfo.

- Works without /proc mounted on modern kernels
- More efficient than parsing /proc/self/mountinfo
- Better detection of bind mounts via statmount()
- Graceful fallback maintains compatibility

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