rpm: fix incorrect expansion of macros with line continuations for args

rpm: fix incorrect expansion of macros with line continuations for args

Macros in RPMs are expanded before line continuations, so when we write

   %systemd_preun foo \
                  bar

What happens is that it expands to

   if [ $1 -eq 0 ] ; then
        # Package removal, not upgrade
        systemctl --no-reload disable --now foo \ > /dev/null 2>&1 || :
   fi
                 bar

which is obviously complete garbage and not what we expected. It is
simply not safe to ever use line continuations in combination with
macros.

Reviewed-by: Laine Stump <laine@laine.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>