This patch adds a TOYBOX_SELINUX configuration option to control both

This patch adds a TOYBOX_SELINUX configuration option to control both
the SELinux commands (such as chcon) and the SELinux-specific options
to regular commands (such as ls -Z).

This lets us #include <selinux/selinux.h> in portability.h.

I've also fixed chcon to insist on being given the a context argument.

This patch also adds -Z to id and fixes id's regular output (-G should
be separated by spaces, non-G output should be separated by commas,
and you don't want a double comma where the egid is omitted from the
list of groups).