Forms position and symmetry

[LamaKing]

The forms to EditUserPwdForm, DelUserForm and RenewForm are created at a fixed position.
If these forms are opened in a "narrow" terminal window, they appear truncated and are unusable.

Moreover, they always spawn at the same coordinates and not in the center of the window. This hurts my need for symmetry.

I'm not an expert on Npyscreen, but I've tried to overcome this bug by retrieving the terminal size from the shell and creating them at (terminal_size-dim)/2, both in x and y. It seems to do the job, even though it's pretty rough.
You can see the cod in the branch Silva, it's just a scratch, any comment in welcome!