mailwrapper: create mailwrapper symlinks for dma(8)

The Dragonfy Mail Agent (dma) can be used with mailwrapper as a
replacement for sendmail(8)

Even if mailwrapper and sendmail are disabled from the build but
dma is not create all the expect symlinks on mailwrapper and make
mailwrapper a symlink to dma, the same way it was done before when
mailwrapper is disabled by sendmail was not.

Discussed with: emaste