[Exim] exim installation annoyance

exim_install insists on installing exim suid, and wants to use chown
and chmod to do it.

To build an RPM this is annoying and unnecessary. It prevents an
ordinary user from building the RPM.

I worked around it by setting inst_uid=$USER, but this is a kludge.

A better solution: an option to just bypass the chown/chmod
completely.