Prepare for network stack as a module

Prepare for network stack as a module

 - Move cr_canseeinpcb to sys/netinet/in_prot.c in order to separate the
   INET and INET6-specific code from the rest of the prot code (It is only
   used by the network stack, so it makes sense for it to live with the
   other network stack code.)
 - Move cr_canseeinpcb prototype from sys/systm.h to netinet/in_systm.h
 - Rename cr_seeotheruids to cr_canseeotheruids and cr_seeothergids to
   cr_canseeothergids, make them non-static, and add prototypes (so they
   can be seen/called by in_prot.c functions.)
 - Remove sw_csum variable from ip6_forward in ip6_forward.c, as it is an
   unused variable.

Reviewed by:	gnn, jtl
Approved by:	sjg (mentor)
Sponsored by:	Juniper Networks, Inc.
Differential Revision:	https://reviews.freebsd.org/D2901