MC's Shift-F3 opens the viewer in raw mode without magic/binary
detection. Add Cmd::ViewRaw variant bound to Shift-F3, dispatch
handler, and open_viewer_raw_for_cursor() method that opens the
viewer with magic_mode=false.
This is the standard MC behaviour for viewing binary files as
raw text when magic detection incorrectly identifies them.
Tests: 1416 pass, zero warnings.
vasilito
74ea610745