capsicum: invocation of new LSM hooks

Places that call fcheck() to convert a file descriptor into a
struct file need to call the new .file_lookup LSM hook.  The
most important instances of this are in the fget() function,
but there are a few other direct users of fcheck().

If a new file descriptor is created from an existing file
descriptor, then any rights associated with the original FD
need to be propagated to the new FD.  The .file_install LSM
hook takes care of this, by potentially changing the struct
file that is about to be installed into the FD table.  This
affects accept(2) and openat(2); for the latter, the rights
associated with the dfd need to be propagated through the
code in fs/namei.c to allow this.

The path walking code in fs/namei.c is also modified to enable
the O_BENEATH_ONLY flag if the process is in capability mode,
or if the dfd is a Capsicum capability.

Signed-off-by: David Drysdale <drysdale@google.com>
capsicum: add syscalls to limit FD rights

Add the cap_rights_get(2) and cap_rights_set(2) syscalls to
allow retrieval and modification of the rights associated with
a file descriptor.

When a normal file descriptor has its rights restricted in any
way, it becomes a Capsicum capability file descriptor.  This is
a wrapper struct file that is installed in the fdtable in place
of the original file.  From this point on, when the FD is converted
to a struct file by fget() (or equivalent), the wrapper is checked
for the appropriate rights and the wrapped inner normal file is
returned.

When a Capsicum capability file descriptor has its rights restricted
further (they cannot be expanded), a new wrapper is created with
the restricted rights, also wrapping the same inner normal file.
In other words, the .underlying field in a struct capsicum_capability
is always a normal file, never another Capsicum capability file.

These syscalls specify the different components of the compound
rights structure separately, allowing components to be unspecified
for no change.

Note that in FreeBSD 10.x the function of this pair of syscalls
is implemented as 3 distinct pairs of syscalls, one pair for each
component of the compound rights (primary/fcntl/ioctl).

Signed-off-by: David Drysdale <drysdale@google.com>