Change test if DEL key is allowed from 'curcol > 0' to 'curcol >= 0', as
deleting forward is reasonable in the first column (and probably
expected by users to work).
Support for DEL was first added in commit b3b43508 (BUG: fix for 6462,
delete key should delete the current char, 2008-08-19). The commit
appears to have copied the original logic from the backspace code so the
old 'curcol > 0' logic was accidental rather than intentional.
