fix TPACKET_V3 initialization (GH #329)

libpcap would fail to initialize on Linux when compiled against kernel
headers with TPACKET_V3 support but running on kernel without:

"can't get TPACKET_V3 header len on packet socket: Invalid argument"

Fix init_tpacket() to respect EINVAL because that is what the kernel
returns for unsupported TPACKET versions.
NFLOG: don't crash trying to filter at link layer

Before:

@ tcpdump -i nflog icmp
tcpdump: WARNING: SIOCGIFADDR: nflog: No such device
Aborted (core dumped)

After:

@ tcpdump -i nflog icmp
tcpdump: WARNING: SIOCGIFADDR: nflog: No such device
tcpdump: NFLOG link-layer type filtering not implemented
Fix typo in pcap_set_immediate_mode manpage
Fix link-type for nlmon.

Before patch:
  tcpdump: listening on nlmon0, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes

After patch:
  tcpdump: listening on nlmon0, link-type 253, capture size 65535 bytes
Add names for DLT_NETLINK

Before patch:
  tcpdump: listening on nlmon0, link-type 253, capture size 65535 bytes

After patch:
  tcpdump: listening on nlmon0, link-type NETLINK (Linux netlink), capture size 65535 bytes
Merge pull request #334 from darkjames/master-netlink-fixes

Fixes for netlink