Add a new system call setaltroot(2).

Currently, using the altroot feature is accessible only via the
set_personality() system call.  It is accessible to user space only if there
is more than one exec domain in the system.  This patch allows using the
altroot feature on systems where there is only one exec domain.

It is possible to work around the issue by adding a dummy exec domain, but it
was rejected for not being very elegant.

If this feature is implemented in userspace, it adds a 16% overhead on a test
case which greps for a single word in the kernel source tree.
Signed-off-by: Zou Nanhai <nanhai.zou@intel.com>
Signed-off-by: Gordon Jin <gordon.jin@intel.com>
Signed-off-by: Arun Sharma <arun.sharma@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>