dhclient: raise WARNS to 4

[asomers]

Summary

dhclient: raise WARNS to 4

Mostly const-correctness fixes. There were also some variable-shadowing,
unused variable, and a couple of sockaddr type-correctness changes. I also had
trouble with cast-align warnings. I was able to prove that one of them was a
false positive. But ultimately I had to disable the warning program-wide to
deal with the others.

[cem]

Looks good. Two questions below

sbin/dhclient/dispatch.c

106

Should this be the _Alignof the pointer type or the struct type?

109

Is this needed with the NO_WCAST_ALIGN Makefile variable?

[asomers]

sbin/dhclient/dispatch.c

106

Good catch.

109

Technically no. But it will be needed if anybody ever reenables WCAST_ALIGN and fixes the other violations.