transport: introduce LogTransportAuxData struct

The purpose of this struct to encapsulate transport specific meta
information that relates to a given read operation. Some transports
(like unix-domain sockets) provide additional information that is useful
for syslog-ng, like the PID of the sending process, etc.

This struct makes it possible to carry this kind of meta information
in a format that allows injecting these values as name-value pairs to
messages. The peer GSockAddr address also got its new home in the struct.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
LogProto/LogTransport: use LogTransportAuxData to pass meta information around

Earlier, syslog-ng didn't have the notion of transport/logproto specific
meta information. The only thing that was passed around when receiving
a message is the address of the peer as a GSockAddr instance.

This patch is changing this, it uses the introduced LogTransportAuxData
struct, which encompasses all information that the LogTransport/LogProto
combination is willing to provide.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
LogReader: feed LogTransport provided aux messages into the newly created message

This patch iterates on all nvpairs provided by the LogTransport/LogProto
part and adds them to the newly created LogMessage.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
transport: make LogTransportSocket a derivable class

This patch publishes the instance struct and adds init_instance()
methods.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
afsocket: added unix domain specific LogTransport derivative
afsocket: use LogTransportUnixSocket for unix domain socket drivers
system: use ${.unix.pid} to overwrite $PID if present

The unix-dgram driver will automatically set ${.unix.pid} if the
system supports SO_PASSCRED socket options, which is the actual PID
sending syslog-ng the message. rsyslog and journald both uses this
PID in place of the PID sent by the peer process and with this patch
so will syslog-ng.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>