Clean up the debug printing in libproc a bit. In particular:

* Don't print any error messages to stderr unless DEBUG is defined.
* Add a DPRINTFX macro for use when errno isn't set.
* Print the error string from libelf when appropriate.
Import pf.c 1.635 and pf_lb.c 1.4 from OpenBSD

Stricter state checking for ICMP and ICMPv6 packets: include the ICMP type

in one port of the state key, using the type to determine which
side should be the id, and which should be the type. Also:
- Handle ICMP6 messages which are typically sent to multicast
  addresses but recieve unicast replies, by doing fallthrough lookups
  against the correct multicast address.  - Clear up some mistaken
  assumptions in the PF code:
- Not all ICMP packets have an icmp_id, so simulate
  one based on other data if we can, otherwise set it to 0.
  - Don't modify the icmp id field in NAT unless it's echo
  - Use the full range of possible id's when NATing icmp6 echoy

Difference with OpenBSD version:
- C99ify the new code
- WITHOUT_INET6 safe

Reviewed by:	glebius
Obtained from:	OpenBSD
Improt pf.c 1.636 from OpenBSD

Original log:
Make sure pd2 has a pointer to the icmp header in the payload; fixes
panic seen with some some icmp types in icmp error message payloads.

Obtained from:	OpenBSD
Import pf.c 1.638 from OpenBSD

Original log:
Some ICMP types that also have icmp_id, pointed out by markus@

Obtained from:	OpenBSD