capsicum: special case for SCTP peeloff

The SCTP peeloff operation is implemented as a particular getsockopt
operation.  However, it is sufficiently special that it has a right
of its own (CAP_PEELOFF), so add special case processing to look out
for this operation (level=SOL_SCTP, optname=SCTP_SOCKOPT_PEELOFF)
and ensure that CAP_PEELOFF is present.