kdump: nicer printing of wait*() ID arguments

Currently, both of these will print pid/uid/gid/sid/jid in hex, which
isn't very helpful for eyeballing. Align more closely with truss(1) and
print these in decimal instead.

ip is likely wider than our ID type, so we do a small casting trick in
wait4(2) to get it sign-extended and rendered properly.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50496