[dwm] setmfact improvements

Hi,

Attached are two small patches to setmfact. The first one removes some
redundancy, saving 5 LOC; the second one reintroduces the MFACT define
that was present (as MWFACT) before, so that you can do setmfact(NULL)
to restore your initial master factor.

Taken together, they add 1 LOC.

Regards,
Peter