Rather than calling mircotime() in catchpacket(), make catchpacket()

Rather than calling mircotime() in catchpacket(), make catchpacket()
take a timeval indicating when the packet was captured. Move
microtime() to the calling functions and grab the timestamp as soon
as we know that we're going to call catchpacket at least once.

This means that we call microtime() once per matched packet, as
opposed to once per matched packet per bpf listener. It also means
that we return the same timestamp to all bpf listeners, rather than
slightly different ones.

It would be more accurate to call microtime() even earlier for all
packets, but microtime() can be costly, so this didn't seem like a good idea.

Obtained-from: FreeBSD (dwmalone@freebsd.org)
Remined-and-Reviewed-by: joerg