capsicum: require rights on socket FD lookup

Add rights_required argument to sockfd_lookup_*().

For sendmsg(2) / sendmmsg(2), CAP_CONNECT is needed (in
addition to CAP_WRITE) if there is a destination address
to send to.  Adjust ___sys_sendmsg() so it takes two
struct file* parameters, one looked up with CAP_WRITE and
one with CAP_WRITE|CAP_CONNECT, and use the appropriate
one according to whether msghdr.msg_name is present or not.
capsicum-test: configure a loopback interface

In the UML test configuration, include a loopback interface
in the minimal init script so that socket tests work.