Declare several parameters/variables constant

Add the "const" keyword to parameters and variables that are never
modified. Most of these were spotted by "-Wwrite-strings".

We cast the second parameter to execvp() explicitly as it expects a
"char *const[]" where it should expect a "const char *const[]"
(according to the documentation, this is due to compatibility reasons).
This should be changed once we come up with a better solution.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/todo.c

@@ -463,7 +463,7 @@ todo_update_panel (int which_pan)
 
 /* Attach a note to a todo */
 void
-todo_edit_note (char *editor)
+todo_edit_note (const char *editor)
 {
   struct todo *i = todo_get_item (hilt);
   edit_note (&i->note, editor);
@@ -471,7 +471,7 @@ todo_edit_note (char *editor)
 
 /* View a note previously attached to a todo */
 void
-todo_view_note (char *pager)
+todo_view_note (const char *pager)
 {
   struct todo *i = todo_get_item (hilt);
   view_note (i->note, pager);