If world is built with a custom sendmail.cf, use it for the distribution

If world is built with a custom sendmail.cf, use it for the distribution
target.  This is the feeder for mergemaster / etcupdate.  This change
makes installworld/mergemaster/etcupdate behave the same regardless of
whether SENDMAIL_MC or SENDMAIL_CF is used.

If you use a custom SENDMAIL_MC/CF in make.conf and excluded it from
mergemaster.rc/etcupdate.conf to work around the conflicts, you may wish
to revert that or change it from 'ignore' to 'always install'.

If you do not use a custom SENDMAIL_MC/CF, there should be no change in
behavior.