Exit if bpf is used in IPS mode
af-packet: warn about BPF filter consequence in IPS mode

This patch add a message to warn user about the impact of using a
BPF filter in IPS mode.
bpf filter: use SCLogError instead of fprintf
Don't try to sniff 'default' interface

Whan running suricata via 'suricata --af-packet', the list of interfaces
was containing the 'default' interface and sniffing it was attempted.
This was not wanted.
unittest to display #784.
Handle the case of pcre combined with a relative content, where pcre has the
set to match from start of line and we discontinue matching on not finding
match.