mount ownership and unprivileged mount syscall (v9)

Andrew, Al,

Please consider adding this series to your trees.

I've been using these patches for a while on my laptop to mount fuse filesystems as user, without any suid-root helpers. The setup is as follows:

- link /proc/mounts to /etc/mtab
- patch util-linux-ng with http://lkml.org/lkml/2008/1/16/103
- remove suid from mount, umount and fusermount
- add a line to /etc/fstab to bind mount ~/mnt onto itself owned by the user
- add 'fs.types.fuse.usermount_safe = 1' to /etc/sysctl.conf

Apart from '/dev/sda2' being replaced with '/dev/root' in 'mount' and 'df' outputs, I haven't experienced any problems.

Thanks,
Miklos

v8 -> v9

- new patch: copy mount ownership when cloning the mount namespace

v7 -> v8

- extend documentation of allow_usermount sysctl tunable
- describe new unprivileged mounting in fuse.txt

v6 -> v7:

- add '/proc/sys/fs/types/<type>/usermount_safe' tunable (new patch)
- do not make FUSE safe by default, describe possible problems associated with unprivileged FUSE mounts in patch header
- return EMFILE instead of EPERM, if maximum user mount count is exceeded
- rename option 'nomnt' -> 'nosubmnt'
- clean up error propagation in dup_mnt_ns
- update util-linux-ng patch

v5 -> v6:

- update to latest -mm
- preliminary util-linux-ng support (will post right after this series)

v4 -> v5:

- fold back Andrew's changes
- fold back my update patch:
o use fsuid instead of ruid
o allow forced unpriv. unmounts for "safe" filesystems
o allow mounting over special files, but not over symlinks
o set nosuid and nodev based on lack of specific capability
- patch header updates
- new patch: on propagation inherit owner from parent
- new patch: add "no submounts" mount flag

v3 -> v4:

- simplify interface as much as possible, now only a single option ("user=UID") is used to control everything
- no longer allow/deny mounting based on file/directory permissions, that approach does not always make sense

v1 -> v3:

- add mount flags to set/clear mnt_flags individually
- add "usermnt" mount flag. If it is set, then allow unprivileged submounts under this mount
- make max number of user mounts default to 1024, since now the usermnt flag will prevent user mounts by default

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