Handle kernels that don't support PACKET_RESERVE.

Handle kernels that don't support PACKET_RESERVE.

If, for some reason, the header files define PACKET_RESERVE but the
kernel fails with ENOPROTOOPT if we use it in getsockopt(), treat it the
same way we treat systems that don't define PACKET_RESERVE in the first
place.  One might argue that this "can't happen", but it apparently did
happen in Debian bug 625443:

	http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=625443