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/config.c

@@ -337,9 +337,9 @@ config_color_theme_name (char *theme_name)
 
   int i;
   short color[NBCOLORS];
-  char *color_name[NBCOLORS];
-  char *default_color = "default";
-  char *name[MAXCOLORS] = {
+  const char *color_name[NBCOLORS];
+  const char *default_color = "default";
+  const char *name[MAXCOLORS] = {
     "black",
     "red",
     "green",