rc.d/sendmail: Fix error with some configurations

The sendmail startup script can run 4 daemons: sendmail, sendmail_submit,
sendmail_outbound, and sendmail_msp_queue. Of the first 3 at most one
can be enabled. There's a run_rc_command call for each and the ones for
sendmail and sendmail_msp_queue run unconditionally. For some rc
commands this triggers warnings or errors when sendmail_enable="NO" or
sendmail_msp_queue_enable="NO". Since d2e7bb630b83 these errors are
propagated and the whole script fails.

Fix this by first determining which daemons are enabled, setting ${name}
and ${rcvar} accordingly, and then always calling run_rc_command
conditionally.

Also replace ${name}.cf with sendmail.cf because ${name} isn't always
sendmail.

PR: 282585
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D47757