sendmail: remove leftovers from mta_start_script and rc.sendmail

Switch /etc/mail/Makefile to use /etc/rc.d/sendmail instead of
/etc/rc.sendmail this switch should have been done 20 years ago.

While here update the documentation to not refer anymore to
mta_start_script

Reported by: Jose Luis Duran <jlduran@gmail.com>