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