daily 150.clean-hoststat: suppress error when using dma

dma(8) does not have hoststat or purgestat, so this script produces
an error from the daily script. We could disable this script, but
that would mean yet another change to switch back to sendmail. Check
for purgestat in mailer.conf before attempting either hoststat or
purgestat.

Reviewed by: pstef, bapt
Differential Revision: https://reviews.freebsd.org/D37712