Make Summon Forest show eligible squares

[Zooty]

It's not very clear when and where you can cast this spell, so failing to cast it should show those squares in LoS where you could cast it. Maybe there should be a special marker for squares near unexplored areas so you don't leak information about their surroundsings.

[bcadren]

Some idea of how many new trees you'll make would also be nice.

[dpeg]

This is true. A patch would be welcome, but probably best to ask on ##crawl-dev before getting involved.