and make it work more reliably in a number of cases that have
traditionally been troublesome. The new behaviour is:
1) If the filesystem can be determined by the fsid or device,
or uniquely identified by the mountpoint, then just go ahead
and call unmount(2) using the file system ID.
2) Otherwise use fstatfs(2) to resolve the path into a file system
ID (checking with stat(2) that it is a filesystem root directory).
Case 2 can potentially block if an NFS server is down, but it can
always be avoided by using an unambiguous specification. It handles
all the hard cases such as symlinks and mismatches between the mount
list and reality. For example, if a filesystem was mounted as /mnt
inside a chroot, it will show up in the mount list as /mnt, but now
you can unmount it from outside the chroot with "umount /chroot_path/mnt".
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