Move IPv4-specific jail functions to new file netinet/in_jail.c
_prison_check_ip4 renamed to prison_check_ip4_locked

Move IPv6-specific jail functions to new file netinet6/in6_jail.c
_prison_check_ip6 renamed to prison_check_ip6_locked

Add appropriate prototypes to sys/sys/jail.h

Adjust kern_jail.c to call prison_check_ip4_locked and
prison_check_ip6_locked accordingly.

Add netinet/in_jail.c and netinet6/in6_jail.c to the list of files that
need to be built when INET and INET6, respectively, are configured in the
kernel configuration file.

Reviewed by: jtl
Approved by: sjg (mentor)
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D6799