Get DLT_MATCHING_MAX right in gencode.c on NetBSD.

Get DLT_MATCHING_MAX right in gencode.c on NetBSD.

Even though libpcap had DLT_MATCHING_MAX defined to 290 in its
"pcap/dlt.h" header, on NetBSD 9.1 gencode.c ended up with the value 275
from /usr/include/net/dlt.h (regardless if the libpcap package, which
defined it as 276 in /usr/pkg/include/pcap/dlt.h, was installed).

This made the current master branch of tcpdump compiled with the current
master branch of libpcap fail some tests, such as:

tcpdump: unknown data link type 284

Move the HAVE_NET_PFVAR_H block and add a comment. This solves the
problem on NetBSD and does not introduce any immediate regressions on
FreeBSD 13.0, Linux or Solaris 11.