'curcol' is an implementation detail of ncurses so other implementations
of 'form' may not have it. The switch-to-previous-field logic only
exists for overloaded requests of REQ_DEL_PREV, so no need to check for
REQ_DEL_CHAR. For REQ_DEL_PREV, check if the field changed and if it
did, change it back.
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.
This converts the CMake license to a pure 3-clause OSI-approved BSD
License. We drop the previous license clause requiring modified
versions to be plainly marked. We also update the CMake copyright to
cover the full development time range.