Use a proper PRNG for GTK askpass.

Coverity complained that it was wrong to use rand() in a security
context, and although in this case it's _very_ marginal, I can't
actually disagree that the choice of which light to light up to avoid
giving information about passphrase length is a security context.

So, no more rand(); instead we instantiate a shiny Fortuna PRNG
instance, seed it in more or less the usual way, and use that as an
overkill-level method of choosing which light to light up next.

(Acknowledging that this is a slightly unusual application and less
critical than most, I don't actually put the passphrase characters
themselves into the PRNG, and I don't use a random-seed file.)

unix/uxpgnt.c

@@ -52,7 +52,6 @@ void uxsel_input_remove(uxsel_id *id) { }
  */
 void random_save_seed(void) {}
 void random_destroy_seed(void) {}
-void noise_ultralight(NoiseSourceId id, unsigned long data) {}
 char *platform_default_s(const char *name) { return NULL; }
 bool platform_default_b(const char *name, bool def) { return def; }
 int platform_default_i(const char *name, int def) { return def; }