bpf: refactor the BPF code and postpone querying of the engine mode

bpf: refactor the BPF code and postpone querying of the engine mode

BPF codebase queried engine mode earlier than it was determined from
the configuration file/command line. As a result it used the default (IDS)
mode where it could've been configured later on to the IPS mode.
This could lead into an undefined behavior as some Suricata modules behave
according to the engine mode.

PF-Ring, Netmap and AF-Packet all shared almost identical code for
determining the engine mode. It was put into one common function.
Omitted the usage of SCStrdup function in PF-Ring module as it is
uppercased during thread initialization phase.

Ticket: #5957