make: force to overwrite symlink

When doing to successive ``make install'' ln will throw an error
that the symlink already exist, so simply force it to overwrite.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
pcap: let netsniff-ng also record pkt timestamp source

With commit [1] in the kernel, we can also store the timestamp
source in the pcap packet header for later analysis. We do this
by splitting the netsniff-ng's u32 ifindex into u16 tsource and
u16 ifindex. Older kernel do not support the timestamp source in
PF_PACKET, so it will stay 0 and is compatible with older
netsniff-ng binaries.

  [1] http://thread.gmane.org/gmane.linux.network/266878/

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
man: add description about different pcap formats

Add a more detailled description of netsniff-ng's -D option into
the man-page.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>