In ip_ctloutput, if the option is at level IPPROTO_IP but not

In ip_ctloutput, if the option is at level IPPROTO_IP but not
recognized/supported, return ENOPROTOOPT (from POSIX 1003.12).  I continued
to return EINVAL if the level was not IPPROTO_IP.  Also, in the latter
case, I added a check to free any mbuf only for setsockopt, not getsockopt,
although that is not strictly necessary (getsockopt passes a pointer
to a null pointer).  From: Mike Karels <karels@BSDI.COM>

SCCS-vsn: sys/netinet/ip_output.c 8.2