Fix Control modifiers for ASCII control keys

- After enabling keyboard modifiers for regular keys
  I inadvertently broke some keys that were previously
  working - in particular ASCII keys that already imply
  a Control sequence, e.g. Ctrl+t is actually an ASCII
  Device Control key. To solve this I've disabled the
  CTRL modifier for ASCII keys below 0x20
- Unfortunately Vim only seems to accept these special keys
  in their single byte form i.e.

      CSI KS_MODIFIER MOD_MASK_CTRL 't'

  is not the same as (and needs to be)

      0x14
- The downside of this is that some sequences are not working yet
  e.g. Ctrl+Enter
Enable modifiers for the Return key

- I've added a mapping from Return to \r to the
  special keys table, theis enables the use all
  keyboard modifiers with this key.