diff(1): reject conflicting formatting options

This matches GNU diff(1) behavior and, more importantly, eliminates any
source of confusion if multiple formatting options are specified.

Note that the committed diff differs slightly from the submitted: I've
modified it so that we initialize diff_format to something that isn't an
accepted format option so that we can also reject --normal -c and -c
--normal, which would've otherwise been accepted because the default was
--normal. After option parsing we default it to D_NORMAL if it's still
unset.

PR: 243975
Submitted by: fehmi noyan isi
MFC after: 1 week