mount: non-setuid (POSIX file capabilities) support

mount: non-setuid (POSIX file capabilities) support

The mount command does not work properly if you replace suid with
POSIX file capabilities. We still need to check for non-root mounts and
the command has to work in very restricted mode for non-root users.

This patch allows you to remove suid bit from mount and umount. Note
that you need a system with filesystem capability support, e.g.
Fedora 10).

# ls -l /bin/mount
-rwxr-xr-x 1 root root 65192 2008-11-09 22:59 /bin/mount

# getcap /bin/mount
/bin/mount = cap_dac_override,cap_sys_admin+ep

[kzak@redhat.com: all the above comments]

Don't bypass security checks when [u]mount uses POSIX file capabilities
rather than setuid root to permit non-root mounts.

Signed-off-by: Geoff Johnstone <geoff.johnstone@googlemail.com>