Support for dynamically loadable and unloadable IP protocols in the ipmux.

Support for dynamically loadable and unloadable IP protocols in the ipmux.

With pr_proto_register() it has become possible to dynamically load protocols
within the PF_INET domain.  However the PF_INET domain has a second important
structure called ip_protox[] that is derived from the 'struct protosw inetsw[]'
and takes care of the de-multiplexing of the various protocols that ride on
top of IP packets.

The functions ipproto_[un]register() allow to dynamically adjust the ip_protox[]
array mux in a consistent and easy way.  To register a protocol within
ip_protox[] the existence of a corresponding and matching protocol definition
in inetsw[] is required.  The function does not allow to overwrite an already
registered protocol.  The unregister function simply replaces the mux slot with
the default index pointer to IPPROTO_RAW as it was previously.