[gpm] Default handlers don't work anymore with 1.20

[Samuel Thibault]

Hi,

Default handlers don't work anymore with gpm 1.20: they are stuck on the
VT where they were run. Worse, if they are started as daemon they just
can't work.  Looking at Gpm_Open() reveals why: the flag < 0 case
isn't even treated in any particular way.  Is there a reason for having
dropped the 1.19 code that was handling the flag < 0 case?

Samuel