netinet*: remove PRC_ constants and streamline ICMP processing

In the original design of the network stack from the protocol control
input method pr_ctlinput was used notify the protocols about two very
different kinds of events: internal system events and receival of an
ICMP messages from outside. These events were coded with PRC_ codes.
Today these methods are removed from the protosw(9) and are isolated
to IPv4 and IPv6 stacks and are called only from icmp*_input(). The
PRC_ codes now just create a shim layer between ICMP codes and errors
or actions taken by protocols.

• Change ipproto_ctlinput_t to pass just pointer to ICMP header. This allows protocols to not deduct it from the internal IP header.
• Change ip6proto_ctlinput_t to pass just struct ip6ctlparam pointer. It has all the information needed to the protocols. In the structure, change ip6c_finaldst fields to sockaddr_in6. The reason is that icmp6_input() already has this address wrapped in sockaddr, and the protocols want this address as sockaddr.
• For UDP tunneling control input, as well as for IPSEC control input, change the prototypes to accept a transparent union of either ICMP header pointer or struct ip6ctlparam pointer.
• In icmp_input() and icmp6_input() do only validation of ICMP header and count bad packets. The translation of ICMP codes to errors/actions is done by protocols.
• Provide icmp_errmap() and icmp6_errmap() as substitute to inetctlerrmap, inet6ctlerrmap arrays.
• In protocol ctlinput methods either trust what icmp_errmap() recommend, or do our own logic based on the ICMP header.

Differential revision: https://reviews.freebsd.org/D36731