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>
capsicum: prctl(2) to force use of O_BENEATH

Add a per-task flag that indicates all openat(2) operations
should implicitly have the O_BENEATH flag set.

Add a prctl(2) command to set this flag (irrevocably). Include
an option to force the flag set to be synchronized across all
tasks in the thread group.
seccomp: Add tgid and tid into seccomp_data

Add the current thread and thread group IDs into the data
available for seccomp-bpf programs to work on.  This allows
installation of filters that police syscalls based on thread
or process ID, e.g. tgkill(2)/kill(2)/prctl(2).