mount-util: add new path_get_mnt_id() call that queries the mnt ID of a path

mount-util: add new path_get_mnt_id() call that queries the mnt ID of a path

This is a simple wrapper around name_to_handle_at_loop() and
fd_fdinfo_mnt_id() to query the mnt ID of a path. It uses
name_to_handle_at() where it can, and falls back to to
fd_fdinfo_mnt_id() where that doesn't work.

This is a best-effort thing of course, since neither name_to_handle_at()
nor the fdinfo logic work on all kernels.