file capabilities: add no_file_caps switch (v4)

file capabilities: add no_file_caps switch (v4)

Add a no_file_caps boot option when file capabilities are
compiled into the kernel (CONFIG_SECURITY_FILE_CAPABILITIES=y).

This allows distributions to ship a kernel with file capabilities
compiled in, without forcing users to use (and understand and
trust) them.

When no_file_caps is specified at boot, then when a process executes
a file, any file capabilities stored with that file will not be
used in the calculation of the process' new capability sets.

This means that booting with the no_file_caps boot option will
not be the same as booting a kernel with file capabilities
compiled out - in particular a task with  CAP_SETPCAP will not
have any chance of passing capabilities to another task (which
isn't "really" possible anyway, and which may soon by killed
altogether by David Howells in any case), and it will instead
be able to put new capabilities in its pI.  However since fI
will always be empty and pI is masked with fI, it gains the
task nothing.

We also support the extra prctl options, setting securebits and
dropping capabilities from the per-process bounding set.

The other remaining difference is that killpriv, task_setscheduler,
setioprio, and setnice will continue to be hooked.  That will
be noticable in the case where a root task changed its uid
while keeping some caps, and another task owned by the new uid
tries to change settings for the more privileged task.

Changelog:
	Nov 05 2008: (v4) trivial port on top of always-start-\
		with-clear-caps patch
	Sep 23 2008: nixed file_caps_enabled when file caps are
		not compiled in as it isn't used.
		Document no_file_caps in kernel-parameters.txt.

Signed-off-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: Andrew G. Morgan <morgan@kernel.org>
Signed-off-by: James Morris <jmorris@namei.org>