Improve static and const correctness.

- Any non-externally-visible declarations should be `static`.
- Casting away the `const` qualifier from pointers-to-const is
  dangerous. All but one instance of this are now correct. The one
  instance where we can't keep `const` is one where toxav code actually
  writes to a chunk of memory marked as `const`. This code also assumes
  4 byte alignment of data packets. I don't know whether that is a valid
  assumption, but it's likely unportable, and *not* obviously correct.
- Replaced empty parameter lists with `(void)` to avoid passing
  parameters to it. Empty parameter lists are old style declarations for
  unknown number and type of arguments.
- Commented out (as `#if DHT_HARDENING` block) the hardening code that
  was never executed.
- Minor style fix: don't use `default` in enum-switches unless the number
  of enumerators in the default case is very large. In this case, it was
  2, so we want to list them both explicitly to be warned about missing
  one if we add one in the future.
- Removed the only two function declarations from nTox.h and put them
  into nTox.c. They are not used outside and nTox is not a library.

1 files changed, 2 insertions, 2 deletions

diff --git a/testing/misc_tools.c b/testing/misc_tools.c
index b667f991..8fdc936c 100644
--- a/testing/misc_tools.c
+++ b/testing/misc_tools.c
@@ -35,7 +35,7 @@
 #endif // TOX_DEBUG
 
 // You are responsible for freeing the return value!
-uint8_t *hex_string_to_bin(char *hex_string)
+uint8_t *hex_string_to_bin(const char *hex_string)
 {
     // byte is represented by exactly 2 hex digits, so lenth of binary string
     // is half of that of the hex one. only hex string with even length
@@ -44,7 +44,7 @@ uint8_t *hex_string_to_bin(char *hex_string)
     // then the last byte just won't be written in 'ret'.
     size_t i, len = strlen(hex_string) / 2;
     uint8_t *ret = malloc(len);
-    char *pos = hex_string;
+    const char *pos = hex_string;
 
     for (i = 0; i < len; ++i, pos += 2) {
         sscanf(pos, "%2hhx", &ret[i]);