Improve types and names for port forwarding configuration

enum conf_port_type is local to conf.c and is used to track the port
forwarding mode during configuration.  We don't keep it around in the
context structure, however the 'init_detect_ports' and 'ns_detect_ports'
fields in the context are based solely on this.  Rather than changing
encoding, just include the forwarding mode into the context structure.
Move the type definition to a new port_fwd.h, which is kind of trivial at
the moment but will have more stuff later.

While we're there, "conf_port_type" doesn't really convey that this enum is
describing how port forwarding is configured.  Rename it to port_fwd_mode.
The variables (now fields) of this type also have mildly confusing names
since it's not immediately obvious whether 'ns' and 'init' refer to the
source or destination of the packets.  Use "in" (host to guest / init to
ns) and "out" (guest to host / ns to init) instead.

This has the added bonus that we no longer have locals 'udp_init' and
'tcp_init' which shadow global functions.

In addition, add a typedef 'port_fwd_map' for a bitmap of each port number,
which is used in several places.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

passt.h

@@ -33,6 +33,7 @@ union epoll_ref;
 
 #include "packet.h"
 #include "icmp.h"
+#include "port_fwd.h"
 #include "tcp.h"
 #include "udp.h"