IPv4: several modifications for multicast routing with PIM

The purpose of these changes is to allow to use the stock IPv4 module
in PIM routers.

Details:

* added IP_PROT_PIM protocol id to IPProtocolId enum

* IPv4: fire change notification about the following events:
    - multicast route not found
    - multicast datagram arrived on non-RPF interface
    - multicast datagram arrived on RPF interface and to be forwarded
    - multicast datagram arrived on RPF interface and has been forwarded

* IPv4MulticastRoute: out interfaces can be disabled
  by overriding isEnabled() in subclasses of OutInterface

* AnsaIPv4: removed unneeded PIM specific routing methods

* PIM modules set the 'OriginNetmask' fields of IPv4Multicast routes:
    0.0.0.0:         for (*,G) routes
    255.255.255.255: for (S,G) routes
  Therefore IPv4 module will use (S,G) route if exists,
  otherwise the (*,G) route for forwaring.

* pimDM,pimSM: PIM specific code moved from AnsaIPv4 to PIM modules
PimSplitter: join interfaces to ALL_PIM_ROUTERS multicast groups
AnsaIPv4: removed copy-paste code from handlePacketFromNetwork()