Make traceroute decode all the ICMP unreachable messages defined
in http://www.iana.org/assignments/icmp-parameters. Thankfully
IANA's list aggrees with <netinet/ip_icmp.h>.

I've tried to do this in a way which is mostly consistent with
tcptraceroute and Debian's version of traceroute. However, sometimes
a letter is used twice by these versions, so I've gone with:

LBL     tcptr   Debian          (chosen)

ICMP_UNREACH_NET !N !N !N !N
ICMP_UNREACH_HOST !H !H !H !H
ICMP_UNREACH_PROTOCOL !P !P !P !P
ICMP_UNREACH_PORT ! !p ! !
ICMP_UNREACH_NEEDFRAG !F-%d !F !F-<%d> !F-<%d>
ICMP_UNREACH_SRCFAIL !S !S !S !S
ICMP_UNREACH_NET_UNKNOWN !<%d> !U !<%d> !U
ICMP_UNREACH_HOST_UNKNOWN !<%d> !U !<%d> !W
ICMP_UNREACH_ISOLATED !<%d> !I !I !I
ICMP_UNREACH_NET_PROHIB !<%d> !A !A !A
ICMP_UNREACH_HOST_PROHIB !<%d> !C !C !Z
ICMP_UNREACH_TOSNET !<%d> !T !T !Q
ICMP_UNREACH_TOSHOST !<%d> !T !T !T
ICMP_UNREACH_FILTER_PROHIB !X !A !A !X
ICMP_UNREACH_HOST_PRECEDENCE !V !<%d> !V !V
ICMP_UNREACH_PRECEDENCE_CUTOFF !C !<%d> !C !C

Graham Wilson is planning to use the same codes in Debian's version.

MFC after: 3 weeks