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/help.c

@@ -100,7 +100,7 @@ static int
 help_write_pad (struct window *win, char *title, char *text, enum key action)
 {
   int colnum, rownum;
-  char *bindings_title = "key bindings: %s";
+  const char *bindings_title = "key bindings: %s";
   char *bindings;
 
   colnum = 0;