Solo, Undead: support 'm' to fill in all pencils.

Solo, Undead: support 'm' to fill in all pencils.

Keen, Towers and Unequal (and Group) already have this feature in
common: pressing m while no square is selected, causes a full set of
pencil marks to be filled in for every square without a real number/
letter/whatever in it. Solo and Undead share the basic UI principles
(left-click to select a square then type a thing to go in it, vs
right-click to select a square then type things to pencil-mark in it),
but did not have that same feature. Now they do.