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/htable.h

@@ -118,7 +118,7 @@ uint32_t                                                                      \
 name##_HTABLE_FIND_BKT(struct name *head, struct type *elm)                   \
 {                                                                             \
   uint32_t __bkt;                                                             \
-  char *__key;                                                                \
+  const char *__key;                                                                \
   int __len;                                                                  \
                                                                               \
   (key) (elm, &__key, &__len);                                                \