Relax chown requirements when check_owner is false

Relax chown requirements when check_owner is false

HCoop delivers into /afs, and the chown will always fail since the
effective unix user and openafs role ($user.daemon) are not the
same. This is harmless in afs space, and it seems reasonable enough to
not care about the chown failing in the general case when exim will
ignore the perms afterward / if the file already exists and it is
appending to it.